Problems editing mesh

I'm pretty new to mesh editing, been watching a million tutorials to try to figure it out. I thought I'd try something simple like shortening a romper. I thought I did everything right, deleted all the vertices from the lower 2/3rds of the pants and appended the nude bottom in to have the legs, but when I upload the edited mesh into Sims4studio, the outline of the pants is still there. I don't know what I'm doing wrong.