I bought a water heater at Lowes. What do I do with the old one?

You can hold on to it till you have your annual yard clean up and put it out front and they'll take it

I agree with Tim, either they'll give you directions to a drop off or on some cases you can ask the plumber/ installer to remove the old one for you.

Call the City of Fresno or City of Clovis they may have a drop off Location