You can upgrade/downgrade by going to your Client Area and clicking on Services->My Services. Next click the license you want to upgrade/downgrade and then the green Upgrade button. Next, click the Choose Product button to select your desired license type. This will automatically update your invoices in our system. (You will receive an invoice for the difference if you upgrade.)
Your upgrade/downgrade will take effect the next time your server communicates with our license server. This happens automatically once every 24 hours. You can force your server to communicate with the license server using the command /usr/local/lsws/bin/lshttpd -r and then /usr/local/lsws/bin/lswsctrl restart.
There's a little extra information on upgrading/downgrading licenses in our wiki: https://www.litespeedtech.com/support/wiki/doku.php/litespeed_wiki:licenses:upgrade-or-downgrade-a-license