How to change chords is a typical newbie concern in the world of guitar players, and a typical answer from an experienced guitarist is, “slow practice and plenty of it”. It is true that practice is the key to guitar playing but in the matter of changing chords there are a few points in the way you practice that will help you achieve your goals faster.
First, in order to practice changing chords you need to find a song that makes use of some basic open chords like Am, D, C or G7. Your sheet music or tab should have the chord changes clearly marked so that you always know while you are practicing where your fingers are supposed to go next.
This is a very interesting video from Clem Buckley, Guitar Teacher on how to work on changing chords.
When you practice any movement involved in playing the guitar, you need to do it with no unnecessary tension. You can’t be completely relaxed while you are carrying out complex movements like guitar playing, but you can make relaxation an ideal to aim for, then see how much muscular tension is needed to, for example, change from C to G7. One way of keeping you muscles relaxed is to move the whole hand when you are changing chords. You play your C chord, then relax and place your fingers in position for the G7 chord while allowing your hand and arm to adjust position as needed.
But when you are changing chords, some fingers will need to stay where they are. For instance, when changing from C to Am you only need to move you third finger. So, while you are keeping an eye on relaxation, you need to also pay attention to how you are going to go about moving each individual finger in your chord changes. You need a strategy that suits the way your body works, and only you can work out your strategy, using good posture and a relaxed attitude as you groundwork.
Once you have worked out how you are going to change chords, you will start your practice. You won’t be able to play fast at first but start with a simple 4/4 strumming pattern using a mix of up and down strokes. After a few days of practicing you will find that you can play in time for some of the time, even though you can’t play fast. Keep playing slowly, aiming to be able to play in time, and your speed will increase as you practice.