Turning my family room into a bedroom

Currently own a 3 bed detached with a large family room. The family room is on the second floor, with windows and a door (secluded just like any other bedroom). It doesn’t have a closet.

I know that 4 bedroom homes are more desirable, and when I want to sell, I want to be able to market my house as a 4 bed instead of 3.

What can I do to legally convert this “family room” into a 4th bedroom?