To manage your shifts, go to Settings at the top right of your dashboard and click Services under Reservation.
If some days of the week, the services hours are different from the other days of the week, then you need to create an additional service specifically for the different days.
For instance, if on Friday and Saturday, the dinner hours are different from the other days of the week, then you need to create a Specific service, for Friday and Saturday evening.
Click Add a service, only select Friday and Saturday, and enter the dinner hours:
Don't forget to Save!
Note: Since you are creating a specific service for Friday and Saturday night, don't forget to untick Friday and Saturday in the existing evening service. To do so, click Dinner in the band, and deselect Friday and Saturday.