1) Log in to your WHM.
2) Look for a link that says "Upgrade / Downgrade an Account" under the Account
Functions header.
3) You should see something like this:
From here click on the account you wish to upgrade or downgrade and click
Modify.
From there you will see a list of all the plans. Click on the plan you wish to
upgrade the account to and click change. After that the account will be
upgraded.
