Professional players should change their strings around once every month; other players should change strings as they hear sounds grow dull or have issues/breakage. You’ll have to loosen the old strings first by turning the knob in the direction the pitch of the string lowers. After the peg gets really loose, you’ll remove the string from the peg by pulling it out and then you’ll slide it back through the hole where it’s inserted into the bridge. Putting the strings back on basically involves the same process in reverse. Although once you’ve restrung the instrument, you’ll need to retune each string; retuning is best done string by string with an electric tuner (rather than your ear).