Yes you can.  After a visit is saved, click on the Tools item on the left navigation, expand Visits and click on Edit Past Visits.  


From this screen you can do a search for your past visits and then click the Edit or Delete links to the right.




Who's Next only allows you to edit/delete visits that were saved within 90 days.  If the visit falls outside the 90 day window, you can still view the visit, but you cannot edit or delete it.