@Anthrazit To replace an existing Maple Cloud document, you need to ensure that the Document Properties used by Maple Cloud match those of the original; without that there won't be a "Publish Update" button in the dialog that appears after sending the book to the cloud. Open the Maple book and inspect the Document Properties (File > Document Properties). The important ones are those that start with "X-Cloud" and "application_type", "authors", "description", "language", and "title", however, you should be able to ignore the X-CloudVersion, since that is incremented by the cloud site. The others must match the original (possibly some could be changed, but its probably safer to change them at the site).
I generate Maple books from a Makefile so don't actually have experience in manually changing these properties, but it should work.
Followup When you first upload a Maple worksheet/book to the cloud, be sure to save the file after doing so. The process of uploading it adds the fore-mentioned Document Properties. If you didn't save it, they won't exist when you go to update. In that case, you should start by downloading the file.