FUSEDMARC

A spray or squirt of graphite into the keyhole solves most sticky lock difficulties. Buy the lubricant in dust or liquid aerosol form.
an emergency, shave a little graphite dust from a pencil lead with a pocket knife or emery board. Work the shavings into the keyway. Then slide the key in and out of the lock several times. Scrape more lead dust if necessary. But whatever you do, don’t use ordinary oil in a keyhole. It will only gum up the lock.
When locks need changing, you can do it. Residential locks come in three main pieces—one knob, one knob with a shaft, and a latch assembly. But before you can replace your particular lock, you need to know what kind it is.
Work with an open door and a screwdriver. Take out the two screw-head bolts in the doorknob flange and disassemble the old lock. Remove any other screws that hold the latch assembly and pull the latch out. To find the size of lock to ask for, measure from the edge of the door to the center of the lock hole. Write this measurement down. Latch configurations differ also. Therefore, to be absolutely sure of buying a usable replacement, take the old lock with you.
Insert the new latch assembly first. Next slide the outside knob into the latch. Rotate it to position the keyhole the way you want it. The exterior knob flange should fit flush to the door. When you have the first knob engaged, slide the interior knob into position. Let the flange holes line up with the bolt mounts. The knobs should now work the latch, if you have the assembly installed correctly. If not, disassemble it and begin again.
As the final step, secure the flange bolts (they have slotted heads and look like screws once they’re installed).