How can you liquefy dry latex paint for use?

Dried latex paint can not be reconstituted. If you have latex paint in a can and some of the liquid has evaporated, but it still has a creamy texture with no lumps, you may be able to add some water or a latex paint conditioner and make it usable, however, that should only be done when you need to touch up and can not get more of the original paint. If you want to repaint an entire wall, then take the old paint to a paint store and they will match it close enough so that you can use it near the old color.