Safari Web Developer Tools - how to edit a cookie?
You cannot use Safari's Web Developer tools to edit cookies (at least not yet?).

Entries in this table are read-only; if you want to edit a cookie’s value, you need to do so with the document.cookie object in JavaScript (you can use the Quick Console at the bottom of the content browser to modify the cookie and see the results update in real time). Pressing the Delete key while a cookie is selected deletes the cookie.

Ref: https://developer.apple.com/library/archive/documentation/AppleApplications/Conceptual/Safari_Developer_Guide/ResourcesandtheDOM/ResourcesandtheDOM.html