I’ve been working with the freeware version of ACF but recently switched to Pro with a developer’s licence, but can’t figure out how to use the license key to validate the plugin.
There should be a menu item under Custom Field => Updates.
The just takes me to purchase the plugin…is there no way to use my license key to validate my current install?
No, you need to follow the instructions here https://www.advancedcustomfields.com/resources/upgrading-v4-v5/. Follow them exactly, especially the part about activating Pro before deactivating acf4, doing the upgrade and then immediately deactivate 4.