In order to edit the content of a booth (the welcome message, button link, or embedded content), you must join the event as either an admin or the representative for that booth.

To provide a booth representative with their access link, go to the "Access Links" tab for that event, under "Configuration". Then, scroll down to the "Booth Representative Links" section:

Make sure you provide each representative with the link to their specific booth, as a Booth Representative Link will only allow user who join with it to edit the specific booth it's associated with.