Thanks for the quick reply.
I could not update, even after making the repository changes.
my version is LINUXCNC - 2.9.0-pre0-2780-g9f6930741
I get the following:
Hit:1 xhttp://deb.debian.org/debian buster InRelease
Hit:2 xhttp://security.debian.org/debian-security buster/updates InRelease
Hit:3 xhttp://deb.debian.org/debian buster-updates InRelease
Get:4 xhttp://linuxcnc.org buster InRelease [28.5 kB]
Err:4 xhttp://linuxcnc.org buster InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3CB9FD148F374FEF
Reading package lists... Done
W: GPG error:
linuxcnc.org
buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3CB9FD148F374FEF
E: The repository '
linuxcnc.org
buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(

manpage for repository creation and user configuration details.
W: Target Sources (base/source/Sources) is configured multiple times in /etc/apt/sources.list:25 and /etc/apt/sources.list:27