You could buy those finger clamp thingies (it's in a V shape and all you have to do is press them together to exercise your fingers) from daiso or shops that sell gym equipment. Just to train the finger muscles. They don't train the dexterity, but the stronger fingers help
You could try this:
Practice changing the chords from Am to G for example but do not strum. Just practice changing first, slowly then increase the speed gradually. (Again don't strum first).
Once comfortable you try both changing and strumming at the same time.