Yes, you can upgrade/downgrade your service at ease only with a few clicks. To upgrade/downgrade your service, please login to our client area and navigate to Services->My Services->Select Service->Upgrade/Downgrade.
Select the new service plan and your upgrade/downgrade would be instantly processed.
Please be noted that at the moment KVM & Windows VPS service upgrades are processed manually which may take upto 24 hours and downgrades aren't supported yet.