You can increase or decrease the disk quota for the cpanel accounts created in the reseller packages using the below steps:
Step 1: Login to WHM > Account Information > List Accounts and look to the far right for the Quota.
Step 2: You can click the + mark in the left side of domain name and click on change quota option.
Step 3: Here you can increase/decrease it to the number you want and click Save.
You may also make changes to the package in case you want to increase/decrease the quota for multiple clients.