If anyone reads this (as it's the top result for googling GLC battery torx): it's a T40 torx screw holding the battery in the front near the middle (not on the right side like on older models like the GLK).
On my GLC, I had nothing to do after replacing the battery: no ESP error, no window controller to retrain, nothing. Even the clock came back on its own (either from the GPS or from the cell network connection).