To edit a single instance of an already scheduled recurring event:

1) Hover your mouse over the appointment and right-click
2) Select Edit Instance
3) Edit the instance and click the Save button

Note: all other instances will be preserved.

To edit all instances of a recurring event:

1) Right-click on the event and select Edit Event or double-click on the event
2) Click the Edit button found in the tools menu at the top of the window
3) Edit the event and click the Save button