As long as the day is not "locked from editing" then you may freely edit your caledar in the same manner that you originally set your calendar. Making edits to your calendar is no different from setting your calendar initially. If the day is "locked from editing" then you may no longer make changes to that particular day. The "locked from editing" option is a feature of the "Availability Locking" system which is an optional feature that organizations may choose to turn on or off.
You can read more about the Availability Locking feature at this support article.