Cutting meshes

Hi!, its my first time using blenderartists.org. I work at roblox making custom cars and custom car rims, I really like the Donk car community and i like to work with donk cars on roblox (As ive said). Im not that good on Blender and i need help to make a car fender bigger. I already cutted the Fender but i need help to make the inside of it bigger (The black part). As you can see here i tried but a part of the interior it’s coming out, and I can’t erase it because when I erase the vertices the inside has parts missing. So i want to know more how can i cut this. The car is a 2016 chevy camaro
image

Select the interior and use bisect to cut it off:

Okay thanks bro :+1:t5:

Help bro i cant select the bisect, it doest shows for me

You need in edit mode and select the target mesh.
Then activate bimesh, your cursor turns into a crosshair then just draw a line and the dialog appears.