Can I, after already exporting an iMovie, re-edit that same movie and update it. If so how, please?

If you have not deleted the iMovie project itself, you can edit and share out as many times as you want from the same project.

If you have deleted the project after sharing out as a movie, you can create a new project and import the movie into the project as one long clip. You then can edit it to a limited extent, such as trimming, splitting, overlaying titles, adding effects, adding additional clips, and so on. However, edits that are already embedded in the movie, like transitions and titles, couldn't be changed. In other words, you can't entirely change the movie to a totally editable clip.

-- Rich