Welcome to the Atlassian Community!
Log into Trello as the owner/admin of the system, and in the main workspace page that you land in, click "settings", and you should get a screen including both a button to upgrade and a tab for it (the button takes you to a trial, the tab gives you a direct cost indications and payment options.
Clicking "settings" should get you something like:
Congratulations to @Laura Holton , our latest winner of Taco Tuesday! And thanks to @Kristján Geir Mathiesen for sharing the picture of Taco having fun with his new friend B...
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