Choosing Your Server Software
The Software tab shows the game build your server is running. Don't Starve Together only ships a single Public branch, so there's nothing to switch between — the tab is mainly there to trigger an update to the latest build. Saving a change here restarts your server.
Your server won't start until you've added a Klei authentication token. See Installing Mods on Don't Starve Together for the required cluster_token.txt setup step — do this before your first boot.
Next Steps
See Installing Mods on Don't Starve Together for Steam Workshop installation and the required server token setup. Take a backup before installing new mods so you can restore your world if one causes trouble.