Hi,
I'm having squealing noise coming from what I believe from Serpentine Belt.It goes away after driving for a while.
What could it be, pully or water pump?
Car has 92.000km.
GLC 63 SUV / 2019
How much will it be to fix?

 

It's the turbo belts. Very expensive to repair. Turbo belts are unavailable worldwide right now. Might as well trade the car in.

It could be the serpentine belt or it could be an idler pulley. Not sure why you think it's a water pump. A bit difficult to diagnose over the internet without audio.
Cost depends on the problem and how you get it repaired. Dealer or Indy.

 

Water pump because it's one of accessories driven by the belt. My initial thought is a pulley.
How hard is it to change the belt and pulleys as far access to it?

 

most likely not one of the accessories...most likely to be the serpentine belt its self or one of the idler pulleys. I've read that the idler pulleys are notorious for going bad...

 

Remove the serpentine and give all the idler pulleys a spin. They should stop reasonably quickly and not spin for an eternity. If you hear one with a rumble that’s probably your culprit.
Using the correct size torx socket on a breaker bar pull the tensioner back to remove the belt. Draw yourself a picture of the orientation of the pulleys or find it on the internet before you start.