A Backup Before a Blog Upgrade
An upgrade is easier to think through when you know what you would restore if the site stopped working. With a WordPress blog, that means keeping both the database and the files. Posts and settings live in the database. The files include your uploads, themes, plugins, and configuration.
Try a restore before the real upgrade. Export the database and copy the site files. Import the export into a separate local database, give the test copy its own configuration, and check that it uses a local site URL. Open an old post with an image, then sign in and look for a recent draft. Those two checks cover different parts of the backup.
Once you can open the restored copy, use it to rehearse the upgrade. This is an exercise for a supported local WordPress installation, inspired by the sort of maintenance a blog owner faced in the WordPress 2.0 era.