On the appointment overview shown on the right side after selecting the desired appointment, click on "Actions" and select "Manage recurring service" from its drop-down.
Once taken to the "Manage Recurring Service" page, click the "Actions" button again and select "Adjust service frequency".
Alternatively, you can also click on the "Adjust service frequency" box under "Recurrence"
Adjust the frequency of the service by looking under the "Repeats" section in the pop-up box. You can also select the day of the week when the service will repeat, under the "On*" section.
The "Starting" and "Ending" fields will be the new starting and ending times of the new recurring schedule.
You will be able to see on what dates the services will land based on the adjustments made to the recurrence.
Hit "Adjust Frequency" to apply your changes.
Another way you can adjust the frequency of the recurring service is to click on the appointment you want to make the changes to, and click on Edit recurring service
This will take you to the page described at the beginning of the article
Frequency Notes:
Appointment options do not include recurring yearly, every 4 months, or any other custom time frame. However, creating a task offers other frequency options.