How do U clean the return registers if they are painted to wall?

You can try using extensions on your vacuum and trying to suck out as much gunk as possible with the screen/grill on. But I would take a sharp box cutter or other blade inside a handle and cut around the perimeter of the screen/grill to cut the paint seal, then unscrew it and go to town vacuuming the inside of the vent.

You could use your vacuum attachment, but I might run a razor blade around the edge so I could remove it. Then you can scrub it with soap and water.

Whoever painted last was lazy - sorry, but true!. Take an xacto knife and carefully score the paint for a clean sharp edge. Continue scoring gently till you have broken the paint seal. Then unscrew from wall, clean and replace.

You have to unscrew them and take putty knife along the edge to try to remove...you might take paint off though so do it carefully

Slide a putty knife in the seam and hammer lightly all the way around.