The operating system running on our Debian-based web servers will become obsolete over time, so over time it will be necessary to completely upgrade the distribution. In this description, we will perform a complete upgrade of a Debian 9 (Stretch) web server to Debian 10 (Buster). There are many features on the server that will also be updated. On the first page of this description, we will perform the pre-upgrade steps to prepare your system for a secure upgrade.

If there is a major change to your Debian system - so you don't just need to upgrade one or two sub-versions - you will get an error message during a smooth upgrade:
This must be accepted explicitly before updates for this repository can be applied. See apt-secure (8) manpage for details.
This troubleshooter will fix this.

When you are not using the latest version of a Linux distribution, oddities may arise. For example, in Debian 9, you may receive a "public key not available" (NO_PUBKEY) error message when updating the backports package. We will remedy this problem in this short troubleshooter.

If you are using custom PHP versions on your Debian server that are updated via the Sury.org repository, you have previously configured this repository, which was still working before, and APT upgraded the packages without any problems. However, an update may cause an error in the package manager and you may not be able to access the latest packages. We will remedy this problem in this short troubleshooter.

When updating our packages, the APT package manager may hold some pieces. This is mostly because a particular program sometimes goes through changes that change the package dependency of the base software. In this case, you would need additional new packages, sometimes ones that require the replacement of other packages. Thus, in some cases, the normal-mode package update does not perform an update to such software, but APT leaves the matter to the user. In this brief description, let’s look at an example of how to update our packages withheld in this way.