When the hot and cold water on your shower is reversed, this is a tell-tale sign that someone did a rush job on your plumbing. There was probably a leak, and the repair person either did not take the time to put the faucet back together properly, or the water was turned off and the repair person did not test the water. In the end, the problem is a quick fix.
Turn the water off at the main. This is your choice: You can work with it on, and it will make the job easier when testing to see if you have assembled the parts back together correctly. Having the water on can also save you time, as you won't have to run back and forth later.
Unscrew the faucet using a small Allen wrench (a small standard screwdriver will also work). Take off the faucet handle and set it aside.
Unscrew the cartridge screws with a Phillips screwdriver and set them aside.
Take off the hot limit stops on the cartridge and change the setting 180 degrees. The hot limit stops should just lie on top of the cartridge. Whoever worked on it previously had reversed the hot limit stop, and this is why you have cold water running instead of hot. With some faucet handles, you have to take the entire cartridge out and flip it.
Screw on the cartridge screws, put back the handle and test it without screwing the handle all the way. If it works right, then you can screw the handle back on.