Our shower seems to only have 2 temperatures: frigid and burning hot. What adjustments do we need to make to have warm water?
Best thing to do is trial and error on the control of your water heater temp. Find a happy medium and warm water should be the result.
I would imagine if you cannot control the actual temperature effectively, then you may want to look at your pressure balancing valve. That controls the ratio of hot/cold water, and over time, they can malfunction for a variety of reasons. It is located where you would expect it to, behind the adjusting know/lever on your shower. But this is not as simple as replacing your fixture. You have to go into your plumbing behind the wall to address this one to replace fully. Sometimes there is a cartridge or removable parts you can clean with CLR cleaner and lubricate with silicone grease and reinstall. That would be where I would start.
If you are only having this issue with that shower and not the other fixtures in the house then replacing the cartridge in the shower valve will fix the problem. The shower cartridge mixes the hot and cold water to give you the desired temperature. You will need to turn the water off to the house , drain the building , remove the trim for the shower , remove the cartridge and then replace it with the same brand and model. Removing an old shower cartridge can be tricky and require special tools. An experienced plumber can perform this job within an hour. I recommend having the work done by a licensed plumber to save yourself the headache and possible repercussions of damaging your shower valve.