What version of Debian? In Bullseye, the version packaged is 2.22.
If you want to keep an older version of your distribution, you could
install 2.22 via the official generic packages from
http://lilypond.org/unix.html. These are pretty easy to use, just
unpack them and run the script
('sh lilypond-blabla.sh --prefix=/I/want/it/here' and I can't
remember if you need sudo; use --help for a list of options).
You don't (need sudo) if you're content to install LilyPond locally for
your user only. (Also you don't need to unpack them first: The files you
download _are_ the shell scripts.)
For example, I have a folder ~/lilypond-versions in my home directory.
If a new release comes out, I just do
mkdir ~/lilypond-versions/2.23.bla/
sh lilypond-blabla.sh --prefix /home/lukas/lilypond-versions/2.23.bla
And @Stefan: If you're using Frescobaldi, that program is perfectly
equipped to deal with multiple versions of LilyPond. Just add the newly
installed package in the LilyPond settings of Frescobaldi by telling
Frescobaldi where to find the "lilypond" binary file. (It's in the bin/
subfolder of the folder you gave as --prefix.)
Lukas
