1. Stop the server before changing files
Do not update or copy the active world while players are connected. End the session cleanly, let the server finish saving, then make the backup before changing the installation.
2. Back up the actual Valheim save directory
Iron Gate's dedicated-server guide documents the default save location under AppData/LocalLow/IronGate/Valheim on Windows and ~/.config/unity3d/IronGate/Valheim on Linux. If you use -savedir, back up that custom location instead.
3. Keep your custom launch script separate
Iron Gate recommends editing a copy of the supplied server launch script because Steam updates can reset the original file. Keep the copied script and its server name, world, password, port and backend settings documented.
4. Update the server and check the backend
For cross-platform groups, confirm the server still launches with -crossplay. Iron Gate says this uses the PlayFab crossplay backend; without it the Steam backend is used and only Steam players can see and join the server.
5. Verify the world before inviting everyone back
Join once, confirm the intended world loaded, check a known base/storage location, then test one crossplay client if your group mixes platforms. Keep the pre-update backup until the group has played successfully.
6. Remember Valheim's own backup controls
The official server guide exposes -saveinterval, -backups, -backupshort and -backuplong. Those are useful rollback layers, but an external copy before a major update still protects against mistakes involving the whole server folder or disk.
Official references
Iron Gate dedicated-server guide · Patch 1.0.15 · Crossplay FAQ
Unofficial guide. Product and game names belong to their respective owners. Always check current official patch notes before changing a server or important save.