Edit center point

i’m trying to make a graphic that swoops upwards fill with color following the swoop. to do this i was going to mask the graphic and have just a big grey square tween up along the swoop by moving its center point to one of the edges and having it turn

only problem is i can’t seem to figure out how to edit center point :fight:

i remember being able to do this simply but i can’t figure out how to do it in this version of flash. help please and thank you