Due to heavy pollution and lots of greenhouse gases, fresh oxygenated air is reducing from atmosphere and lots of Carbon dioxide is getting generated. There is however nitrogen present in much amount. Humans are trying to save themselves by learning to breathe Carbon Dioxide exactly like trees or separating Carbon and Oxygen.....
Can evolution change the most basic thing: "Breating Oxygenated air" to breathing a convenient air, whatever is available?
You're talking about evolution to respond to this. Evolution is a grindingly slow process and won't work as a fix like this. You'd need to consider genetic alteration by artificial means.
However humans actually deal with these problem by using technology. We're more likely to create a controlled living and working space and employ breathing apparatus that to try altering ourselves genetically. The technical challenges of the former are trivial compared to the challenges posed by the later.
Is it even possible that even can adapt these changes in them?
Your entire body chemistry is dependent on Oxygen. A creature not using oxygen would be entirely different from a human (or anything else we know of).
The side effects of trying to change to another breathable gas would be basically to require every single human biochemical process to be altered. We simply lack the capability of doing this and it's way beyond anything we would expect to be able to do for the imaginable future.
If yes, can anyone with the help of some science (I don't know), please provide the probability or the positive possibility of the same?
As I've said, humans would try and find a (simpler) solution for the short term : adapting living quarters and using filters and artificial climate control (i.e. inside structures).
Trees and $CO_2$
As you mention trees and $CO_2$ note that the more likely scenario is using some form of cellular organism to generate Oxygen for us in vast facilities. Why alter humans when we might more easily create cellular life that can do exactly what we need - generate Oxygen for us ?
The Human Condition
Humans don't generally respond to problems they create by changing humans or human behavior. They "solve" the problem by finding the simplest (and most cost effective) fix using gadgets and technology that lets us carry on doing what we were. Altering human biochemistry is the very last thing that would be done.
Psychologically most humans would be violently opposed to such a physical change.
Sociologically I think even if some people did allow such a drastic change, they'd become a social grouping or minority, possibly hated by most other people ( "Carbos" leaps to mind as a term ). Humans do so like to hate people who are different. People who are different with an advantage would be really top of the list for dislike.