Here's a simple rhythm for you use for practicing changing between the open C Major and a minor chords. The chords are shown in traditional music score and also in tablature. Simply play four down-strums of your C Major chord and then four downstrums of your a minor chord. Go slow and get the timing of your changes perfect.
Down strum arrows are shown as a guide.
If this is the first time you have come across these chords on guitar you will soon realise that you only need to move one finger to swap between these two chords. That may seem like only a small move as far as playing is concerned, but when you get into music theory there is a reasonable change in the music structure.