You need to make sure that there are no pieces between the rook (or tower as you call it) and king. You also need to make sure that the king is not in check. You then move the king two squares towards the rook and then move the rook to the other side of the king (but next to it). Look through some other players' games and you'll see how it is done.
HOW TO SWITCH
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Also, if your king has moved beforehand, you can no longer do this (it is called castling if you want to look it up)
HOW TO SWITCH TOWER AND KING?