To remove a UPC shower cartridge, follow these steps:
1. Turn off the water supply to the shower.
2. Remove the handle and trim plate to access the cartridge.
3. Use a cartridge removal tool or pliers to grip and twist the cartridge counterclockwise.
4. Pull out the old cartridge and replace it with a new one.
5. Reassemble the handle and trim plate, then turn on the water supply and test for leaks.