Superfix is correct. The original TOC has a small nut on the inside. Its a bit fiddly to fit and best to do it before fitting the new lamp assy .
I take it that it’s the lamp assy fitted in the top right back corner ?
The original lamp is held in with ‘sprung clips’ around the lamp body. Theres 4 IIRC . You will need to access it from the back and push the clips in whilst gently pushing the lamp assy out into the oven cavity. Best put an old towel down in the oven base as unless you have arms like a chimpanzee or an assistant , you can’t reach inside whilst pushing the old one through.
You’ll also find that you’ll prob have to bend the lamp connections on the new part ( 424077 ? ) to allow proper connection of the cables and to stop them touching the casing.