How do you change a paint color that I already bought?

If the purchase was recent a lot of stores offer to replace the paint if you don't like the color. They don't want dissatisfied customer and want your business. Most of returned paint ends up on the "oops" shelf discounted.

Been there done that and William is right.

We have all done that, and generally people like me purchase the oops or "save me" paint. Hopefully you still have the receipt!

Take it back to the store, they will usually alter it. Some colors can't change much but sometimes a shade or two either way is all that's needed.

Any place that does paint.. Some will charge a few dollars to retint it!!

If you take the original paint swatch with the tint numbers, and the swatch with its paint numbers that you thought the color would look like, sometimes it is only a matter of adding more units of one or two numbers to lighten or darken it.

It may not be possible, depending on how intense the color is, but if the store where you bought the paint at has a no-return policy on tinted paint, it is worth a go to try to salvage the problem this way.

Store has no return policy because it is gone. It moved

You can mix it with other paint, but always try it on a small amount first. you can add white to lighten it, or black to darken it. Certain colours mixed together will make a muddy brown.

http://www.housepaintingtutorials.com/custom-paint-colors.html

I know Home Depot will retint paint! Just be careful, because nothing new will match it because with retinting you lose the formula.