You can easily upgrade your current Reseller hosting plan by opening your Hosting → Manage:
Once there, click on Upgrade button to upgrade your current plan and increase your overall plan limits:
This upgrade will increase your Reseller’s disk space, cPanel accounts, inodes and so much more! To modify your packages inside the reseller account, check this amazing article.
For full root access to WHM you would need to check our VPS plans (cPanel license is needed separately).