If you have reached capacity and need additional resources on your plan you will need to upgrade the plan from the My Account area, to do this complete the following steps.
Once logged in, under My Services, select the server you wish to upgrade
Click on 'Upgrade Package'
You may be displayed a warning advising that, as part of the upgrade, you may need to restart the service. Acknowledge this message and continue by clicking Proceed.
Find the package you wish to upgrade to and select a billing cycle
Press 'Choose'
Select a Payment Method and click 'Continue'
You will now see an order confirmation screen
You will then be issued an invoice for the upgrade, the upgrade will take place once the invoice is paid.
Check here to check if your server will require a reboot.