Its important to relax and not rush when changing between chords. Often your fingers may land in wrong positions if you try to such the process. Its important to establish how you'll switch between two chords before you play in time.

Also, you can use a passing chord. This occurs on the last beat before a chord change. On this beat you can remove your fingers from the previous chord position and play the strings open as you take the position of the next chord.

For example: If you're playing an 8 beat strumming pattern with a G chord, and in the next bar you switch to a D chord, you can play the last beat open before you switch to D. The chord change will be much smoother that trying to switch chords from one beat to the next.

Hope this is helpful :)