When you say replace do you mean that the bulb has just blown and you want to replace the bulb? If so I believe you need this type of bulb
G4 Halogen CapsuleTo access the bulb it looks like you may have to unclip the glass cover. I have similar under my kitchen cupboards but the glass cover rotates on mine.
They are generally pretty cheap fittings and the 10W Halogen bulbs generate a lot of heat and this can cause issue with internal connnections which you may need to look at if the bulb hasn't blown. I'm assuming that you have other working downlights which will confirm that the transformer is OK.
Newer LED downlights are better but you'd also need to change the transformer as well as your current one may not be compatible with LED lights.
An explanation of this is can be found below if you want to go down that route. It is something I ought to do as my existing halogen downlights are a bit unreliable due to contact issues caused by the heat that's been given off by them and frying the contacts. I've put off doing it only because
I'm lazy access for routing wires for me is a little awkward.
Transformers for 12V LED lighting