How can I upgrade from PSE 2021 to PSE 2023?

To upgrade, go here, click Buy now, and choose License upgrade from the drop down:

https://commerce.adobe.com/checkout/payment/?items%5B0%5D%5Bid%5D=4041B60C94E03F1E8D32D94ED6EC122F&cli=adobe_com&co=US&lang=en

Ventura requires the latest version of PSE (PSE 2023). Previous versions basically wont work.    

 I have my serial number but when I enter it, I'm told it's already used (yes: by me!).

By @philippereichert

Please go to Adobe Customer Service and only ask to have your activations reset. (Make sure to sign in to your Adobe account first and allow popups and cookies on your browser. You may not be able to see the chat window if using a VPN.):
https://helpx.adobe.com/contact.html?rghtup=autoOpen will automatically open the chat window.
Type "Agent" in the chat box to bypass the chatbot and be connected to a person.

You may also be able to contact Adobe by phone:
https://helpx.adobe.com/contact/phone.html

or Twitter:
https://twitter.com/AdobeCare

Migration doesn't work for Adobe apps.

Use the Cleaner Tool to uninstall all Adobe apps and then do a proper installation of PSE 2021:

https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

Then go to your Adobe account and download PSE 2021 if you don't have the installation file.

You will need your serial number:

https://helpx.adobe.com/download-install/kb/find-serial-number.html