Changing the bulbs in under cabinet lights?

I have under-cabinet lighting in my kitchen, which are round glass with a central 20w halogen bulb in a circular plastic case. No make or model numbers I'm afraid.

4 light bulbs now need changing and I can't work out how to do it. I have 3 spare lights in boxes, but none have instructions, and I can't work out how to do it from the spare.

Can anyone hrlp?

Many Thanks
Jioanna