No Automated Mod Manager
Valheim doesn't have a Mods-tab browser on this platform. There's no built-in catalog or one-click installer — anything beyond the vanilla server has to be set up by hand, and there currently isn't a verified, tested procedure for doing that on a hosted Spawnly server.
How Valheim Modding Actually Works
BepInEx is the standard, widely-used third-party mod loader for Valheim — most Valheim mods are built to run on top of it. Spawnly's server process doesn't include any special handling for BepInEx or its launcher, though, so we can't currently confirm a working install path that survives restarts and updates cleanly.
The File Manager (Files tab) is the only way to upload files to a Valheim server here. If you want to try installing BepInEx yourself, treat it as advanced and unsupported — we can't guarantee it will keep working through server restarts or Valheim updates.
What We'd Recommend
If mods are the main reason you're hosting a Valheim server, weigh that against the lack of a verified path here before committing to a modded setup. Vanilla Valheim — on either the Public (Main) or Public Test branch — is fully supported and works exactly as expected.
Next Steps
Take a backup before experimenting with any manual file changes — see Managing Backups & Restoring Your Server. For branch details, see Getting Started with Your Valheim Server.