Are you using MapTiler Desktop Start and do you want to upgrade your plan? You can easily upgrade to the PLUS version inside the application itself.
Go to Account→ License key, click on the button Upgrade to PLUS.
The cost for the upgrade is only the price difference between the START and PLUS plans.
Check the current prices at MapTiler Desktop Pricing.
In case you are currently using the FREE version and you are keen on upgrading, read the article about License activation article.
Are you using FREE, START, or PLUS, and do you want to go PRO? If so, please contact MapTiler and request a DEMO session at https://www.maptiler.com/demo/