I know how to add bookmarks into a DjVu file:
But how do I edit existing bookmarks? Do I need to re-insert a modified bookmark file? If so, is there a way to extract the bookmarks into a text file, edit the text file, and re-insert the bookmarks into the DjVu file?
Yes, you need to extract existing bookmarks to a file, edit it, and then set it back (old bookmarks will be overwriten):
# extract bookmarks
djvused -e "print-outline" my.djvu > bookmarks.txt
# edit bookmarks.txt
# set bookmarks back to document
djvused -s -e "set-outline bookmarks.txt" my.djvu
You may also find some GUI tools that will allow you to edit bookmarks, but basically they do the replacement under the hood.
sudo apt install mono-runtime libmono-system-windows-forms4.0-cil, then download and unpack app archieve, Then grant "start-linux" script execution rights with chmod +x ./start-linux. Then launch it with ./start-linux. The executable is a .NET code so it can be launched on Linux with mono in some cases.