If your Metro is like mine and my other belt drives here's the basic procedure:
Jack rear frame up so that your tires are off the ground.
Remove side drive belt cover.
Disconnect your handle/lever from brake so that the rod is hanging by the handle.
Take tension off of the large spring located towards the lower back of frame connected to drive belt pulley.
Spin your belts off of your driveline, hopefully you can do this, I have on all of mine. Pull on your belts while turning the tire to remove them.
Remove any mechanism that is guiding the belts onto the rim (kind of hard to explain)
Depending on which side tire you are on, you will probably have a nut or a snap pin or snap ring holding your rim on. Remove whichever one you run into.
Your rim should pull right off.
Hopefully I explained this well enough for you to understand, if you need clarification or have any other questions please ask. Good Luck