The last thing you want is to lose your hard work due to a server glitch, a faulty update or simple negligence (Yes, we've seen save files accidentally take a trip to the recycle bin, and that hurts....) so, lets prevent that. Let's go through the steps on how to backup your server, and what each particular file does.
So, you'll need to locate your world first. If this world is hosted on your PC, you'll need to go to C:\Users\<<My Username>>\AppData\LocalLow\IronGate\Valheim\Dedicated to find it. You'll find 2 files in the "worlds" folder - one is a .fwl file, the other is a .db file. You may also have .fwl.old and .db.old files. For Southnode clients these are located within the File tool under the Saves folder. But what are all these files?
The .fwl file is the server seed. This seed references the DB file name, and is the first file to be loaded. This .fwl file will be named the same as your world. So for example if your world is named "Odins Hammer" your .fwl file will be named Odins Hammer.fwl
The .db file is what most would consider to be your "actual" save file, as it contains the database for all the things that are happening and have happened within your world. This is the larger of the two files, and is the most "fragile". This .db file will also be named the same as your world. So for example if your world is named "ByThor" your .db file will be named ByThor.db
Anything ending in .old is a backup made by the server itself. You should keep these around just in case you need to roll back a save to an earlier point in time.
After locating these files, you will need to stop your server - NEVER under any circumstance attempt to make a copy of the files whilst the server is running. Your backup will likely become corrupted (particularly the .db file). Make sure your server is turned off gracefully (ie. if you're self hosting, use the Ctrl+C combo on the console when on the system itself to gracefully interrupt the server) - this will trigger a save, which is good! After shutting the server down gracefully you can now make a copy of the files, and put them in a safe place. We recommend a USB drive, or cloud storage to keep these babies safe! We recommend for larger servers with lots of work put into them that you backup at least once a day.
After performing a backup you can restart your server and continue playing whilst being assured your hard work and struggle is maintained for as long as you need!