If you current hosting plan qualifies for an upgrade, you can do so from the client area: https://my.hostmantis.com/clientarea.php
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 from you current hosting plan
After payment, your account will be instantly upgraded to the plan you choose. Upgrading your hosting plan does not affect your account data, etc.