Step 1 Soak and Clean
Do this by soaking and cleaning it in a large bucket of warm water and paint remover. Allow to dry. If you don't do this immediately, paint can leak onto the roller frame. It will also dry which will make the cover more difficult to slide off.
Step 2 Slide Off
The majority of roller covers will slide over the top of a cylinder which can also be removed. Detach the cylinder and slide the roller off using a small utility knife to prize the cover away from the cylindrical surface. Use a small amount of force and gently pull the cover away. You can just swap the cylinder for a new one with a new cover if the old one is excessively worn.
Step 3 Install New Cover
Roll the new cover up and peel it down over the surface of the cylinder. Make sure it is nice and snug. Alternatively, click the new cylinder with a cover onto the roll frame.