Organization Users - The Super Administrator will be able to purchase additional storage for the organization and allocate it to specific users. Login to mail.zoho.com as the Super Admin, and click the Subscription link on the top of your mailbox. Buy the Add-On that's necessary for your organization from the listing. Learn more.

Personal Account Users - From your inbox, go to Settings >> General Settings >> Mail Accounts >> Buy now >> Select the suitable storage. Learn more about purchasing additional storage for personal accounts here.