Well, it's probably not as much of a problem as you fear. Brickmold is simply the exterior trim around a door or window, a part of the casing that extends over the siding and covers any gaps between ...
Remove exterior trim, along with enough exterior siding or finish to reveal the flange in a sash-less window; if the window has a sash, it will already be removed. Unscrew or pry the flange away from ...