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How to case a window
Remove any existing trim.
If you're redoing a window that already has trim installed, you'll need to get rid of the old before you can put in the new. Use a utility knife to score the caulk lines around the edges of the window trim, including the top piece, sill, and apron.
Then, pry up the old casing with a crowbar or the claw end of a hammer. Cut and pry carefully to avoid damaging the window jambs or any part of the surrounding wall.