In the Current Plan panel click the Next Billing Date / Next Billing Period value (labelled Next Renew Date in the editor), pick a date and save. The date cannot be in the past ("Renewal date cannot be in the past.") because a back-dated renewal would make the subscription overdue and re-charge it on every run. Saving also sets Day To Renew to that date's weekday. To move only the weekday, change Day To Renew in Renewal Settings. The renewal window (buffer) is not editable per subscription – it comes from the plan's Buffer Days to Charge Before Renewal.