How difficult is it to replace wifi ic?

I guess if you have to ask the question, then yes, it is too difficult for you to do :>).

This is by no means an easy repair unless you have the proper tools and some experience working at this scale. It certainly shouldn't be the first thing you try. There are plenty of videos on youtube that make it look easy but these folks are generally using multi-thousand dollar micro-soldering workstations under stereoscopic magnification.

Do you have a hot air station (not a heat gun)? Are you experienced in removing micro-BGA IC's from densely packed iDevice logic boards?

If you don't feel comfortable doing this yourself, then look for a repair shop that does micro-soldering repairs. It shouldn't be a very expensive repair.