I chose the wrong file when importing a manuscript. Do I have to start over?

Not from version 1.9.8. In the import preview, click "Replace file" next to the file name (there is a second one at the bottom of the page), pick the correct file, and the chapter list is replaced with the chapters of the new file. Everything you already filled in stays: author, genre, sub-genre, language, author profile, series choice and translations. The title follows the new file unless you had typed your own. If you cancel the file dialog, nothing changes.