There are two ways you can modify the disk quota for a cPanel account in WHM. This will depend on if you have set up packages for your users or if you manually set the resources for each account when you first created them.
Packages:
If you have set up different packages for your customers, the easiest way to achieve an increase in disk quota for each customer is to simply edit the package in WHM and increase the disk quota. Once you have made the change, every account under that package will have the new size.
You can edit the package from within the ‘Packages’ section of WHM - or simply type package into the search field just under the WHM logo in the top left corner, then select ‘Edit a Package’ from the menu.
Single Account:
If you only want to modify a single account :
1. In WHM, go to the ‘List Accounts‘ section (either via the homepage or search box)
3. Click the + sign next to the domain you would like to change
4. Select ‘Change Quota’
5. Enter the new quota for the account and click Save
You can also refer to the cPanel documentation here:
https://docs.cpanel.net/whm/account-functions/quota-modification/