If you haven't yet bought extra storage for your subscription, you can do that.
- In the admin center, go to the Billing > Purchase services page.
 - At the bottom of the Purchase services page, in the Add-ons section, find Office 365 Extra File Storage, and select Details.
 - On the product details page, select Next.
 - If needed, choose the base subscription, then enter the number of gigabytes of storage you want to add.
 - Select Check out now.
 - On the How does this look? page, verify the number of gigabytes of storage you selected, review the pricing information, and then select Next.
 - On the Complete order page, verify the total. If you need to make any changes, select Edit order. If the order requires a credit check, select the check box. When you're finished, select Place order > Go to Admin Home.
 
Increase or decrease your extra storage:
If you've already bought extra file storage via the Office 365 Extra File Storage add-on, you can use these steps to increase or decrease the extra storage space for your subscription. You can reduce the storage to as low as 1 gigabyte. To remove all of the extra storage space, contact support.
- In the admin center, go to the Billing > Your products page.
 - On the Products tab, select the subscription that contains the Office 365 Extra File Storage add-on.
 - On the product details page, in the Add-ons section, select Manage add-ons.
 - In the Manage add-ons pane, from the Add-on list, choose Office 365 Extra File Storage.
 - In the Quantity text box, enter the number of GBs of storage space that you want for the subscription.
 - Select Save.