Reconstruct a plane and keep UV intact This is probably simple, I want to remove some unused space since it takes up lightmap space in my game



Basicly I want to remove the red part but keep the green withsame texture cordinates so I need to correct UV somehow. 

Mesh looks like this in blender 2.8



So basically I want to shrink down to the 3D cursor, but also remap UV.

edit: I solved it just because it was even measures. The wall is 4 meters tall and the wall I want is 1 meter. So i moved down the vertices, now the UV is wrong. Than I moved the top vertices in the UV mapping down from 1.0 to 0.25. Must be a more bullet proof way of doing it?
Use a knife or loop cut to break the square in half, then delete the extra vertices.