No one can do it.  Even if they all knew *EVERYTHING* about our modern technology, were perfectly orginized, and had the resources of a kingdom backing them up it couldn't be done in 50 years.

The problem is that of building tools.  I can't just build a computer with the help of a blacksmith.  I need *very* exact tools to fit 10^8 circuits per square centimeter of chip.  To build automated tools that exact I would need, in turn, other special tools, which I would need specialized tools to build etc etc.  Even doing everything perfectly it's just not possible to go from middle age tools to computer age tools in 50 years, it takes too long to build all the intermediate, slightly more resigned, tools in succession.

There is also, as already pointed, we are a very specialized society.  No one person knows everything, or anything remotely close to everything, about technology.  Even if you sent all of NASA, YALE and Harvard, and MENSA back in time you would still be loosing the vast majority of our knowledge.

EDIT: as BrianDHall said kidney transplants are not quite as doable as I thought.  However, many other surgeries would be plausible, though less effective, If a surgeon trained in the older surgery techniques were sent back.  Surgery is more about knowing the body and how it works, it is less dependent on specialized tools (relatively speaking!!).  The biggest issue would be lack of proper anesthesia, limited ability for sanitation (basic boiled water can be used, but it's not the same as a proper sterile operation enviroment), and lack of drugs to control infection and/or body immune response(though as I said a basic antibiotic could theoretically be created in a few years, penicillin was just moldy bread after all!) .  This means a lower success rate of surgeries, but if it was life or death then a surgery with risk of infection is still better then death!  Which surgeries are possible and which aren't really comes down to rather they needed specialized drugs pre or post operation, and how hard it is to operate on a patient that is not under anesthesia (ie that could move while your doing a percise cut) I would assume.  I admit I'm less knowledgeable in this area.


Some cool things that *could* be theoretically done by a lone person with any STEM education, intellect, some support from the 'natives', and perhaps an interest in science (but no very unique or specialized skills beyond high school) are:

1) curing lots of diseases.  Penicillin is just mold on bread.  Smallpox can be cured by having everyone make good friends with cows and their polka-doted milk maids.  MANY diseases could be cured by teaching people sanitation and to not defecate into their drinking water.  Others can be survived by getting rid of rats and misquto infestations.  If someone recognized a particular disease they could help to cure it.  The idea of a vaccination *may* be possible to generalize to any virus (give me my high school knowledge, 5 years, and a kingdom's support and I could try a few tricks to make vaccines that might actually be partially effective, though I feel sorry for my first test subjects).  All of these would take trial and error, not a do-right-first-time thing, but could possible be done.  For that matter basic sanitation like having people who are doing surgery or delivery babies wash their hands first could save a massive number of lives!

2) teach scientific method and advanced math.  None of these change life directly, but the concepts are what make the Renaissance and industrial revolution possible.  You can rapidly accelerate human development with these concepts

3) teach high school science.  Really, Newtons first book on Newtonian physics came out right after the middle ages had officially ended (just barely), so they don't know physics yet!  Calculus was discovered a century after that!  Germ theory was figured out sometime between the two.

4) Teach evolution.  This is one science that is very easy to explain, it's not based off of any complex math and could be understood by anyone who has an open mind without much other scientific basis.  Of course managing to reveal it in a way that doesn't make you the enemy of religious orthodoxy is another issue, remember the catholic church was VERY powerful for much of the middle ages and was not afraid to excommunicate or worse those that they didn't like

5) Get people to believe you are from the future just with the clothes off your back (and phone in your pocket).  Imagine what a watch would do.  Imagine what a scientific calculator or laptop would do!!  Sure they would run out of power eventually, but you would have convinced everyone who you are by then and used them for good effect.

6) helped design gunpowder and the gun.  It's entirely possible someone would remember most of the gunpowder's ingredients.  You won't know fully how to build a gun or even gun powder, but the principles and ingredients given over to well-funded experimenters of the day could perhaps make a gun happen?

7) cure scurvy.  Scurvy is interesting, the 'secret' to curing it was discovered and forgotten many times.  Since we didn't know what it was about fruit that cured scurvy experiments with using cheaper fruits (that didn't have vitamin C), or cooking or filtering our storing it in such a way that the vitamin C was lost occurred many times in an effort to cut costs

8) 'discover' the New World.  That one isn't even hard to do.  "hey king guy, I'm the dude that can make people move and talk on a magic screen by pressing button and just cured a deadly disease.  I tell you that if you send 3 ships in that direction with lots of food they will find something awesome."

9) correct stupid misconceptions like blood letting that were getting people killed and maimed for no reason.

10) tell the 'future' by remembering your history.  Though keep in mind that the more one does this the more they change history to make this not work.  Plus we all tend to be taught rather inaccurate history in high school, someone trying this may find out that real-life wasn't quite like the bastardized euro-centric version we were taught if they aren't careful.