How long have you been playing? If you've only started recently, it's common to have a bit of trouble with finger strength in your pinky and ring finger, and thus have trouble changing chords. Just start by playing the chord changes slowly, or at whatever speed is necessary for you to get them right, and use a metronome to slowly increase speed. It will mean a workout for your fingers, and you're sure to get tired, but it will be worth it if you persist. Stop playing if your fingers start to hurt too much, though.

Hope that helps a bit. :)