As long as you use good quality watercolor paint, you can reactivate them by moistening them again. All you have to do is rinse the brush thoroughly, remove excess water from its bristles and gently rub the area with circular movements. The dry paint inside the tube can be reactivated with water because each pigment particle is encapsulated in Arabian gum. Try adding some water back to the tube, little by little, until you have the desired consistency.
Alternatively, you can separate the tube from the dried paint and use it as if you were using half a mold. If the color has solidified in the watercolor tube, it is not possible to return it to its liquid form. But it can be used as a type of coloring for pans.