First you will need to confirm that the information for the unit is correct; billing amount and next bill date under the member's Unit Info section.
Then you will use Unit Actions > Generate Membership Bill for XX. The bill for the next cycle will be generated and you can find it under the members unpaid tab. NOTE: The newly created dues will still show they are scheduled for the original bill date.
You will want to select the upcoming dues from the Unpaid tab, click Pay/Schedule and then make a partial payment by selecting the Enter Different Amount option and entering the amount of the credit you wish to apply.
Next, select the Account Credit as the Payment Method and check the box to do a partial payment in the amount of the Account Credit.
This will apply the amount of the Account Credit toward their upcoming membership draft and only draft the remaining amount (if any).