You can easily modify your Reseller account packages by opening your WHM.
Once there, click on List accounts:
Select the username that you want to upgrade/downgrade, expand the list by clicking on the + sign, and select Change Plan:
Now select one of the packages to upgrade/downgrade your client and click on Upgrade/Downgrade:
Green means you can select the package to this user;
Gray means the current package to this user;
Red means this package is not available (more information below).
Upon confirmation, the account package will be changed 😊
You can modify your package limits by going to Edit packages under the Package section.
Your red-colored package may be blocked because you set unlimited resources or the resources are higher than you have available in your current cPanel reseller plan.
Your main account (the account configured for your plan) quota or features can’t be modified, the package is set by the system upon the plan configuration.