@hughashton Hello! I do not think Mastodon has an “edit button,” per se. I also use Toot!, and when I tap into a toot that I have composed, I see a three-dot menu icon just to the left of the “Reply” button.
In that menu, one of the options is “Delete & re-draft.” That option will delete your toot and put its contents back in your composition box. You can edit it there and re-upload. This will change the timestamp on your toot. So, if you start editing content from months ago, be advised that it’ll be added back at the top of your timeline.