To make an appointment type change the sex of a patient on completion. So for that you first go to the system (gear) menu and select Appointment Management:
You then select the switch to the Appointment Types tab:
Here you find the appointment type you would like to have alter the client sex upon completion and click on it and then scroll down to the section with the checkboxes.
Next just check the Alteration Appointment option and then click save. Once that is complete the appointment will automatically mark the patient as altered once completed.