Removing dirty sheets. Changing the bedsheets is an easy chore that you should take on every two to four weeks (or less if the sheets become dirty sooner than that). There are three parts for bedsheets: the top sheet, the bottom sheet and the cat. Both of these sheets should be changed at the same time, but don’t change the cat. Don’t leave one sheet on and change the other. Leave the cat alone. But beware, a cat is fascinated by the front-load washing machine. Please check the washing machine before closing the door.