If you already have Start license activated, you can easily upgrade your license to Plus from inside the MapTiler Desktop application.
Go to File -> License... -> Upgrade your license to PLUS.
For the upgrade, you will pay only the difference between the Plus and Start licenses, which is currently 120 USD.
If you are using the Free version of MapTiler Desktop and you want to upgrade to a higher version, read the License activation article. If you want to upgrade to PRO, you need to download and install different binary file.