How to adjust temp. on a Delta shower faucet?

Take off the round cover-plate near the water spigot, and inside there will be a metal tab or adjustment device that can be turned to change the amount of hot water that is sent to the shower head.

If it's a Delta Monitor, remove the handle. You will see a small pointer on the cap of the valve module, and an arrow indicating which direction is hotter.

Gently pop the cap up and turn the cap a few notches in the hotter direction, then push it back down. Put the handle back on and try the temperature now.

NOTE: if your shower won't get hotter no matter what you do - especially if it's just tepid - it's possible your anti-scald device in the valve module has failed. If that is the case you will need to replace the valve module, as the anti-scald device is not repairable. That, or your water heater itself has failed. To eliminate the water heater as a source of the problem, just check how hot the water gets at a faucet. If it's still good there, your problem is in the valve.

Good luck with it.