Remove the action from the piano. To do this, loosen the set screws holding the fall board in place and lift the fall board out of the piano. Then remove the large screws, underneath the action bed that holds the blocks on each side of the action, and take out each block. Slowly and carefully pull the action from the piano. Set it either on a towel, to prevent scratching, that you have placed on the closed lid of the piano or on a table. The hammer line is on top of the action and the work to replace any hammer can begin.
Look to see if each hammer has a number either stamped by the manufacturer or penciled in by a piano technician. If not, number every hammer starting with the largest one on the left side. This prevents any mix-up if more than one hammer is removed. Back out the flange screw of the hammer to be replaced and lift the entire hammer unit out of the hammer line. The unit includes the hammer, shank, knuckle (underneath the unit), drop screw and butt flange.
Find a piano supply house that sells individual hammers rather than just complete hammer sets. Write up an order to replace the hammer that you have taken out. Include the hammer number, length of the straight side of your piano, manufacturer and serial number. Place the completed order and the entire hammer unit from your action in a box and mail it to the supply house. The returned shipment should include not only the replacement hammer but also the hammer you sent to be replaced.
Set both hammers with their connected shanks and flanges side by side for comparison. Look for the following to be the same: hammer size, hammer angle in relation to the shank, distance from the flange screw hole to center pin and distance from the flange screw hole to the knuckle center. If these measurements are the same, then install the new hammer by placing the flange on the action rail and screw it down.
Manually move the new hammer and the adjacent hammers up and down. Check for alignment with the existing hammers. There should not be any touching of the hammers to the right or left as the new hammer travels. If there is any touching, back out the flange screw and place a paper shim on one side of the screw to correct the problem. When finished, check the flange for any wobbling movement and, if any, tighten the screw snugly. Slide the action back into the piano; replace the side blocks and fall board.