Within Settings > My Services > Group & Order, rearrange the order by clicking and holding the 4-directional arrows icon to drag-and-drop a service or group to another position:

Multi-service bookings can be enabled/disabled within Settings > General by selecting the booking process type:

If enabled, to configure the service sequence, within Settings -> My Business, click [Manage Services] against the barber/therapist to show their service list, and rearrange the order by clicking and holding the 4-directional arrows icon to drag-and-drop a service to another position:

As an example with the above screenshot, if a customer books:

The sequence of their appointment would be Haircut & Wash > Back & Sides > One Grade All Over.