I have found that for the files in the format Djvu (which are opened with Djvu reader) there is a function that allows you to rename bookmarks
So I just convert PDF to Djvu. The Djvu are handled more quickly by Boox M92, but the Djvu reader has fewer features and does not support the Table of Contents...
Edit: the TOC are supported by Djvu Reader, only that the virtual key to access the summary is not in the main bar but in menu / tools / summary (very uncomfortable)