Where to place licence to upgrade to paid version?

Hi folks, I have upgraded my free Avast Antivirus program to paid on 3 pc’s. I have the licence number etc but cannot find where to put it to update the program. I am getting regular reminders to ‘upgrade’ but find nowhere to update it. Any advice on this please?
Thanks in advance for any info. Cheers, Jen.

Which Avast…? (Pro/IS/Premier)