Prior to showing the unit to any new applicants, the unit has to be in a state of repair and cleanliness that meets the standard of your other units.
If the repairs you need to make are minor, you can usually do them while the current resident is still there. Then, as long as the unit is clean and presents well, you can begin showing the unit to potential new residents and arrange for a move-in date that’s quite soon after the current resident leaves.
However, if the unit needs a significant amount of repairs, you may have to wait for it to be empty before you can do the work to make it presentable.