brought some paint at resale store want it another color?

Then buy another color.  Then you have that paint and the other color.  You will find someplace to use it, even if that means giving the walls another coat.

I once bought a big 10 gallon bucket of bright purple paint at an auction for 75 cents.  I took it to a paint store and asked them if they could make it turn white. It took 2 days to get the purple paint out of my hair and I never could get it out of my shirt.

Your options depend what color it is, and what color you want.  I often mix my own and add more white, or tint with universal stainers. The only downside is if you run out, you can't mix it again.

 I bought a 5 gallon pail of slightly off white paint at the RESTORE.

 Took it to HOME DEPOT and they colored and remixed it for me. When I asked the clerk for the bill she said it was "NO CHARGE" !!

 It was a brand of paint they sell and she incorrectly assumed I'd bought it there.

 Bur seriously, and reputable paint store should provide this service for a minimal charge.

This is a very tricky thing to do. Depending of the current color there may be limits on what color you can change it to. In any case it is best to experiment with very small samples so you don't mess of the whole can as you try to change the color.

Buy paint in color you want.

Well, buy a sample size of a paint tinted like you want and mix with the paint you bought.  Take the paint to a paint store and let them look at the color, figure out what needs to be added to turn it like you want, and make you up a batch to mix into your paint.  May be the paint store people can do that.  Now for all of the trouble you are going through saving a few bucks on a cheap paint, you could have bought very good paint and got exactly what you wanted.  20 years.

You can add tint to change the color, but the color it is now will dictate what colors you can change it to.