Please read here: Self-hosted Nextcloud 25 is out!
Note: if your Nextcloud version is lower than 24.0.10, you’ll have to follow each upgrade guides, starting from your version (for example for NC 23.xx: upgrade-to-24.0.8.7 followed by upgrade-to-24.0.10.7, then upgrade-to-25.0.8.19 md).