If you need to change the service plan for a customer by upgrading or downgrading their plan or changing which add-on service plans they have, you may do so using the following steps:

1. Log into Plesk using your reseller credentials.
2. Click Subscriptions on the left-hand menu.
3. Place a checkmark next to the subscription you wish to modify.
4. Click the Change Plan button.
5. From the New service plan drop-down box, please select the new service plan you wish to use.
6. In the Add-on plans section:
   1. Locate any add-ons you wish to add, and select add from the drop down box next to them.
   2. Locate any add-ons you wish to remove, and select remove from the drop down box next to them.
7. Click OK to complete the changes.