Hi Lorn and welcome to the forum. If you add your location to your profile you will get better targeted replies. I suspect you are not based in the UK like me. My GLC 220d Sport does not have Torx screws securing the battery clamps, just a 13mm AF nut (1/2" AF which is 12.7mm is a snug fit but fits nonetheless). I popped off the engine cover, unclipped the pollen filter housing and had a look under the battery terminal cover. Perhaps petrol engined models have a different arrangement.
Because these cars have so many sensors and monitoring systems, I would just confirm with the dealer there will be no convoluted system reset procedure required once you reconnect the battery. I assume the car won't be stored in a place where the trickle charger could be connected up with the battery still in place. Anyway, here's a photo of my battery clamp arrangement.
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).