A Peerless single shower head has a mechanism inside of the handle that will change the output temperature of your water. It's a useful feature if you have kids and want to protect them against accidental scalding. This feature is fairly accessible as well. There is a dial on the end of the mixing valve which turns to change the temperature. All the owner needs to do to access the dial is take the handle off.
Pop the cap off the end of the handle, if your Peerless shower has a cap on the handle. Beneath this cap is the handle screw. Unscrew it with a Phillips head screwdriver. If there is no cap, just find the set screw and unscrew it. Pull back on the handle to pull it off the faucet.
Locate the rotational limit stop dial, which is a round part on the end of the mixing valve. The dial is attached to the stem, the rod on the mixing valve, and it controls how far the handle turns in either direction.
Push the dial to the left, or counterclockwise, to lower the temperature of the shower. Push the dial to the right, or clockwise, to raise your shower's temperature. Adjust the dial in small increments. Test it by placing the handle back on the mixing valve and turning the water on.
Leave the water on for several minutes and test the temperature. Turn the dial to change the temperature as necessary. Replace the trim and handle once the temperature is perfectly set.