Upgrading/Downgrading your hosting plan with HostMantis:
Click "My Services"
Click "Active" to the right of the hosting plan you want to upgrade
From the "Actions" menu on the left, click "Upgrade/Downgrade"
From there you will see a list of hosting plans available for upgrade/downgrade from you current hosting plan
After payment, your account will be instantly upgraded to the plan you choose. Upgrading/downgrading your hosting plan does not affect your account data, etc.