If you have a recurring appointment series and want to change the days for the series you can edit the recurring appointment from the Edit Appointment section or the recurring appointment list report.
How to navigate: Appointments > Calendar > Click on Appointment to Edit > Edit Appointment > Recurring Appointment
In the Edit Appointment screen click on the pencil icon to edit the recurring appointment days the appointment repeats on.
In the dialog box that opens make the changes you want by selecting or removing the days from appointment series.
Once you have made the changes click on the Save Change(s) button to save the edits.
Note: You can also make the changes in Recurring Appointment List Report in the reports section.