First, make sure your Debian 10 system is fully up to date:
$ sudo vpn-maint-update-system
Reboot after this to make sure all is still well.
Follow the instructions here.
These are the official Debian upgrade instructions. You can and SHOULD follow
that document and read all sections and perform the steps when necessary. Do
not remove the "non-Debian packages" (4.2.2) as all packages are also
available on Debian 11 and will be upgraded without trouble. You do need to
pay attention to "Changed security archive layout" (5.1.3) as the security
updates will require a different deb line in /etc/apt/sources.list.
Pay attention the the cleanup as well (sections 4.7, 4.8). When updating the
repositories (section 4.3) also make sure you update
/etc/apt/sources.list.d/eduVPN.list and replace buster with bullseye,
i.e.:
$ echo "deb https://repo.eduvpn.org/v2/deb bullseye main" | sudo tee /etc/apt/sources.list.d/eduVPN.list
After the update is complete, PHP will not be properly configured yet, this is because the version changed from 7.3 to 7.4 and is part of a different Apache configuration. You have to manually re-enable PHP:
$ sudo a2enconf php7.4-fpm
In case you also modified the PHP configuration options, as suggested e.g. here, you need to reapply those as well and restart PHP, to restart PHP:
$ sudo systemctl restart php7.4-fpm
Restart Apache:
$ sudo systemctl restart apache2
Run the following scripts to make sure all is in order:
$ sudo vpn-maint-apply-changes
$ sudo vpn-maint-update-system
This should all run without any error and without asking any questions! Reboot your server after this and make sure everything still works. Try logging in to the portal, connect with a VPN client.