To add/remove a Service to/from an existing Reservation, go to the Bookings screen and tap on the Reservation to open it

To Add a Service:

Scroll down to the Services section and tap Add Service

Select the Service, the Employee, and edit the Timings (see What is ‘Processing’ and ‘Finishing’ time?) if necessary

Tap ADD

The new Service will display as a Booking Tile, and will also be listed in the Reservation pop-up:

To Remove a Service from an existing Reservation:

Tap on the Reservation in the Bookings screen, scroll down and long tap on the service you wish to remove, then tap Delete: