Je me fais un petit mémo pour la prochaine fois.
Les commandes :

# sudo -u www-data php /usr/share/nginx/nextcloud/updater/updater.phar 
Nextcloud Updater - version: v29.0.0-1-g9293bcb dirty

Current version is 29.0.4.

Update to Nextcloud 29.0.5 available. (channel: "stable")
Following file will be downloaded automatically: https://download.nextcloud.com/server/releases/nextcloud-29.0.5.zip
Open changelog ↗

Steps that will be executed:
[✔] Check for expected files
[✔] Check for write permissions
[✔] Create backup
[✔] Downloading
[✔] Verify integrity
[ ] Extracting
[ ] Enable maintenance mode
[ ] Replace entry points
[ ] Delete old files
[ ] Move new files in place
[ ] Done

Continue update? [y/N] y
...
Update of code successful.

Should the "occ upgrade" command be executed? [Y/n] Y
...
# sudo -u www-data php /usr/share/nginx/nextcloud/occ maintenance:repair
# sudo -u www-data php /usr/share/nginx/nextcloud/occ upgrade
Nextcloud is already latest version
# sudo systemctl restart nginx.service
# sudo systemctl restart php8.1-fpm.service

Les dernières commandes sont pour ne pas avoir le problème : « Downgrading is not supported and is likely to cause unpredictable issues (from 29.0.5.1 to 29.0.4.1) »

Misère.