Sadly Valheim is not offering a direct way to do automated Backups, so we need to use a little workaround.
Navigate in your dashboard to "Restart Planner".
Now click on the "+" it opens a new window, in which we can define the desired Restart times.
First we choose if desired "Daily", which means that on every day a backup will be created on the time we choose. In the next step we define our desired restart time.
Start offline server - Means that your server will be started if its offline, when the reinstallation time is met.
Now we click on "Save".
You could setup a planned restart all 6 hours, what means that all 6 hours a automated backup will be created.