Then open the /etc/apt/sources.list file with your text editor and replace each instance of stretch with buster and buster/updates to bullseye-security. If you have other source-list files under /etc/apt/sources.list.d, you must update those sources too.
You can check the Debian version by using the following command:
lsb_release -a
If you are on Debian 11, you are ready to install YunoHost !
Warning from @hugogogo when doing this procedure that could be useful for others
I’m on a dedicated server, and when i did the reboot step sudo systemctl reboot, i lost definitively my ssh connection, I cannot connect anymore. It’s possible that some changes have been done during the upgrade to the authorized_keys file, I don’t know, anyway now my connection is rejected. Maybe if i had not forbid password access i could have still use this method, I don’t know.