All you need to do is pull the hard shell or cover just in the opposite direction of the head and remove the head instantly. Thereafter, install the new head. In the case of an eraser roller, find out the wringing lever. It generally exists at the joining point of the head and mop stick.

If you find it difficult to remove the head of the mop easily, then you need to give a twist to the handle in a counter clockwise direction. You can unscrew it to remove the head of the mop and the replacement procedure will be just screwing up the new mop. You can also check the wingnuts and screws at the bottom of the mop handle.