We just had a new hot water heater installed and now the water pressure is low and one of the showers don't have hot water the shower just get luke warm please help

You most likely stirred up dirt in your pipes while working on your heater. Dirt traveled through pipes and clogged up your shower heads and aerators. Remove shower head, clean them, run bit of water w/o shower head installed. Also, remove aerators on your faucets, run water w/o them installed, and reinstall. You should be fine. If not, then dirt got into your cartridges and you will have to remove them . It that's the case, get back to us and we will tell you how to do that.