you need much more practice. To get your hands in the mood to change chords what you could do is on every stroke of a chord (say its C) lift your hand off the chords and put them back on, this will give you the practice of lifting your hands off to change the chord. Once your comfortable with that then try going from C to G.
Do note that certain chord changes are easier than others purely based on the position of the hand. For instance C to G is not difficult, nor is C to E. Aminor to D is also another.
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