The easiest approach would be to mix a little water to thin it back down, but this may not work well if it is very hard. An alternate solution is to use water based latex thinning solutions used for paint sprayers. Latex paint is an emulstion binder system, meaning the non-water soluable acrylic latex particles are suspended in a water solution. Once the paint dries, the latex particles stick together, and then the dried paint is no longer water soluable. Given this, once the paint dries it is not possible to reuse to the same effectiveness as it was designed. As long as it is thick but still fluid you should be able to thin it down enough for touch up by repleshing the water content.