Getting the chrome rings off is a bit tricky. They are crimped on and must be carefully removed it you intend to reuse them. TomHom states above that the rings are still available from VDO so once they are in hand the rings could be considered disposable and removed easily, though care should be taken to not damage the glass.
For me, I'll round off the edges of one side of a small screwdriver to create a flat crescent shape so there won't be sharp corners to dent the edge of the ring as it's pried outwards from the gauge body. It will take a long time with many minute small prying movements traveling around and around to open the chrome ring enough to remove it.
Then, replacing the ring, it will have to be slowly crimped down into place going around multiple times. I'm sure VDO has a device the rolls the edges of the chrome ring into place in short order, but by hand it will take much longer.