If the water temperature of your shower water is either too cold or too hot, your shower mix valve may need to be fixed. The mix valve controls how much hot water is released into your shower. You can fix temperature problems by adjusting this valve. It is a fairly easy process that requires common household tools and should be accomplished in under an hour.
Take off the hot water shower handle. There may be a screw on the end of the handle that needs to be removed before the handle will come off.
Find the large valve stem. It should be directly behind the area where you removed the handle. There will be a screw attached to the valve stem. This screw controls the shower mix valve.
Turn the screw counter-clockwise to release more hot water and clockwise to release less.
Test the valve to see if the water temperature is to your liking.
Reattach the shower handle and replace the screw that holds the handle in place.