I have hot water in the sink but my one water handle in the shower does not get hot only warm. How can I increase the water temperature?

Most common problem is a clogged mixing valve or cartridge on the hot side supply of the shower mixing valve. Easy enough to clear if you can help us determine what valve you have.

Try this!
Shut the water off to the valve and open it up. Remove the cartridge and check the inlet ports for debris and trash.
Now turn the hot water back on to flush out the supply and open valve body, Reassemble and test, Got hot water now? Let me know, Tom