I have 10/2 wire existing, I purchased 10/3 to rewire the new water heater. Black, White and Ground are OK. What do I do with the red that is not needed

If it is a straight 240VAC, you only need the 10/2-Black hot, White hot(make sure to mark it black), ground. If it is, for whatever reason, a 120/240? the black and red are hot, the white is neutral and then ground. If you have already put in the 10/3, use black hot, red hot, and ground, just cap off the neutral. No installation manual? What does the data plate show?