Drifting

Hello, does anyone have information about changing geran?
Yesterday I went to post office to renew my roadtax, but because of the roadtax renewal space was full (at the back of the geran letter) so the post office guy said I cant renew it in post office, I have to go to jpj to change to new geran letter, my question is what is the process of changing the geran? What should I prepare and how much it will cost me? Does anyone experienced this before?

Really appreciate your help,
Thanks...