If I may, this isn’t really the same topic as “how do I buy SonarQube”, as you already bought the license apparently 
To answer your question, follow the doc here: Upgrade Guide | SonarQube Docs, under Changing your Edition. Basically, you need to download and install the Developer Edition (binary is here).
Then you’ll have to send your ServerID to your Sales rep to get the license key, as ServerID will change after upgrade.
Carine