Mopping a dirty floor probably isn't your favorite chore, but sometimes it is a necessary task. When the head of the mop gets dirty, mopping becomes futile work as you simply push muddy water around on the floor. Sometimes the mop head is so worn and dirty than dunking it back in the water for a rinse doesn't help much. When this happens, you'll need to change the mop head to a new one.
Wring out the mop, and stand it up on its handle so you can easily reach the head. Many janitor mops have a wing nut on a bracket that helps hold the mop head in place. Loosen the wing nut and spin it off completely. This should allow you to lift a metal bar that was locked against the mop head. Alligator-type mops that clamp down on a mop head may also have a wing nut that needs to be loosened to free the head from the clamp. Plastic quick-change heads may have an adjustment knob that can be loosened and a plastic clip that unlatches on the side so the head can slide out. Whichever type you have, lift the dirty mop head off the mop, and dispose of it.
Remove the new mop head from its packaging, and find the center panel. On most mop heads, there are strands hanging down on both sides with a flat area in the middle where the mop grips the head. Once you have located the center panel, hang it over the rod or slide it into the space in the quick-change. Half of the mop head should be hanging on one side of the mop and half on the other. The center of the head should be balanced on the mop itself.
Replace the wing nut or adjust the knob on the quick-change version, and hand-tighten the clamp until it is snug against the mop head. The clamp will hold the mop head in place. If the attachment bracket had any other clips meant to secure the mop head, make sure they are back in their original positions.
Flip the mop back over, with the mop head on the floor. Dunk the mop into a bucket, and continue mopping the floor. If the head works loose, you did not adequately tighten the wing nut or clamps.