How can I change an Event with a 'Public Event' category into an Event with a 'Group Event'?

Hello

Some of our members forgot to create an Event from the Group (ie 'Group Event') but create an Event directly from the Event menu (ie. 'Public Event')
You can see both cases on the attached file.

How can I change the 'Public Event' category to the relevant 'Group Event' with a link to a specific Group?
From the admin dashboard or using DB tables?

TIA
Henri