Both donors and admins (on behalf of donors) can edit the date that an existing monthly gift processes right from within the Donor Dashboard. Admins will use the View as Donor tool, but the process is the same as if a donor was to make the change themself.

From the Donor Dashboard or View as Donor tool, select the Recurring tab then choose Edit.

In the Edit Recurring Donation window, choose a new Next Donation date, then save your changes.