Chris at eSpares says...

Most microwave doors come in three main parts.The metal door frame, The plastic outer door (this is usualy clipped over the frame but can also be screwed from the inside). And finally the inner door skin or choke, This will normally clip in place and is of a brittle plastic which breaks easily as you try and remove it. start by removing this first. If you want to change the 'outer' you will probably need the inner as well.