For Server, log in as an admin and go to Admin -> Licence and click on the text box to paste the licence key in over the trial one.
For Cloud, you don't get a licence, you just accept the terms of moving from trial to paid and it's automatically done for you.
Your question makes me suspect that you've set up a Cloud based trial, decided that you like it, and then gone and bought server licences instead of clicking the "continue to paid service" type option. If this is the case, then you need to decide whether you want Cloud or Server versions, and if you need Cloud, email "sales at support dot atlassian dot com" and see if they will allow you to transfer the money for your server licences to your Cloud account to pay for the first few months.
If you had a trial all along, to have a paid subscription all you have to do is to enter your credit card information. Do:
If you have purchased a license elsewhere, then it's a JIRA license and, as Nic say, you need to contact support.atlassian.com to ask for a refund (if possible) and then add the information.
Jaime Kirch da Silveira
Atlassian Cloud Support
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