Remove any wall covering such as paneling or drywall. If you are removing paneling, use a flat bar to gently work the paneling loose at the joints.
Cut the wall studs in half using the reciprocating saw. Once you have the studs cut, gently work the two halves back and forth to remove them from the wall.
Remove any protruding nails by cutting them with the reciprocating saw. The nails that attached the studs to the top and bottom plates should remain in the wood. It would be impossible to pull them out, unless you remove the entire wall.
Cut the studs to the right length. Measure from the top plate down to the bottom plate. Make sure the top plate isn't sagging; if it is, leave about a 1/4-inch extra on the cut.
Replace the wall stud by tapping the new ones into place. Place the top on its mark and gently tap the bottom over to correct mark. Check and make sure the wall stud is plumb using the hand level. Toenail two wood screws at the top and two at the bottom.