I have tried a number of different versions of Content Cloud 12 including latest and once populating a link propery you cant go back and edit it, only option is to remove and re-add another link. In previous versions their was an edit option (button) next to the property
Steps to re-produce on Alloy:
In CMS 11 there was an edit option i.e.
Optimizely have logged this as an Internal Bug:
"I have created a case CMS-25519 with the development team. I will update you once I get a reply from them."