Humans are already very good climbers, how to make them better is going to to depend on how you want them to be better and how much you are willing to change them. Really tool use can make for very good climbers once humans can make rope everything else is details.
To be slow low risk climbers humans are pretty good. You would want to make the fingernails more claw-like although not too much to grip smaller cracks or ledges. Stronger hands will help although making them larger will not, longer hands help chimps because they are climbing trees which have round branches, also if you change human hands very much tool use will suffer.
The other big downside humans have is mass, being large runs counter to being a good climber, because the forces you need to generate, resist, and absorb increase exponentially. Making humans smaller is a simple solution although this has downsides in other things.
Make the feet more handlike, the heel is very bad for climbing, preferably they will be able to rotate much more as well, imagine being able to flip your food around the way you can your hand.
The biggest place we suffer is in climbing downwards, we suck at that. A longer neck will help a lot (although then you want to add a tail for counter balance because our heads are heavy), we have a much harder time climbing down because we have a hard time seeing down in the direction we are going while climbing.
If you want something more like mountain goats and mountain cats, fast moving, you have a much harder time, humans are really bad at that. Quadrupedal is much better than bipedal for distributing forces when moving fast and non-linear on rough terrain. They need to be able to generate strong forces and absorb the impact of long drops, we are too top heavy, our legs are too weak, and too much stress is pushed through the legs and back, so you need to make those stronger without making them heavier.