To increase the resources and the amount of web domains that you can manage on your Multidomain Hosting plan, you have to upgrade your current plan to a higher one.
Upgrading means replacing your current service with a new one providing you with many more resources than the one you are using at the moment, which cannot suit your activity's needs anymore.
To learn more about the upgrade procedure, we suggest you to check our dedicated guide on "How to upgrade your Hosting plan".