Here's what I get when following the directions outlined on this page: https://boinc.berkeley.edu/linux_install.php?os_num=4&build=stable

# sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 40254C9B29853EA6
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.MsGFypOXmZ/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv 40254C9B29853EA6
gpg: key 40254C9B29853EA6: "Vitalii Koshura (BOINC DEB and RPM repos key) <[email protected]>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

If I then follow the rest of the instructions, an old version of BOINC gets re-installed, presumably because the repository was not properly added.

Which version of Mint and which of the Ubuntu or Debian instructions are you using? The current version of Mint is based on Ubuntu22.04 so if running latest Mint, that is the one you should be copying commands from.

In Ubuntu and I presume Mint as well,

Warning: apt-key is deprecated.

is just that, a warning and not an error message and certainly on 24.04 (Ubuntu the testing version installs fine.

Here is the version:

NAME="Linux Mint"
VERSION="21 (Vanessa)"
ID=linuxmint
ID_LIKE="ubuntu debian"
PRETTY_NAME="Linux Mint 21"
VERSION_ID="21"
HOME_URL="https://www.linuxmint.com/"
SUPPORT_URL="https://forums.linuxmint.com/"
BUG_REPORT_URL="http://linuxmint-troubleshooting-guide.readthedocs.io/en/latest/"
PRIVACY_POLICY_URL="https://www.linuxmint.com/"
VERSION_CODENAME=vanessa
UBUNTU_CODENAME=jammy

I will try again with the instructions for Ubuntu 22.04 when I have a moment, and document the outputs of each command more thoroughly.

Please disregard, the new version did install at some point but perhaps the old one was still in memory, even though I had purged the old version. A simple reboot fixed the problem.

Please disregard, the new version did install at some point but perhaps the old one was still in memory, even though I had purged the old version. A simple reboot fixed the problem.

Interesting. I have always if using the same method as the original installation be that package manager or compiling from source been able to update without a reboot. It would be interesting to know if that is something slightly different in the way Mint handles things from Ubuntu or what?