Replacing recessed medicine cabinet, uneven frame?
Use a reciprocating saw to cut the fasteners holding one of the sill/fire-block boards in place. Run the blade right through the drywall; making a slot, right where the two boards meet. The top board would be the ideal choice, as it can't fall inside the wall, when the fasteners are cut, that hold it in place.

If a cripple stud is in the center, trim one of them 1/4" shorter.

Trim the drywall to match the new opening.

Toenail the sill/fire-block into its new location.

Patch any damaged drywall from cutting the old nails/screws.

A circular saw will more easily make a straight cut, but will also remove more wood. If using a reciprocating saw, just take your time and don't get in a hurry.