Need just a little bit lower gear for those big climbs? The Wolf Tooth GCX sprocket replaces the stock 42T on your XX1 or X01 cassette. Shifting performance and durability matches the stock part but you will have a 5% lower gear. Similar to reducing your front chainring size by 2 teeth.
To remove stock 42T: Gently pry with a screwdriver between 36T and 42T at each pin location. You will need to go around the cassette 2-3 times, moving the cog just a little each time.
To install Wolf Tooth 44T: align large hole (see laser marked triangle) with large pin on cassette. Gently squeeze each of the 18 pin locations together with a Channel Lock type pliers until all pins are bottomed out.
Detailed step-by-step instructions are available here.