Choosing Your Server Software
The Software tab for DayZ only offers the Public branch — there's no alternate to switch to. Any change here still requires a server restart, and Spawnly confirms this before saving.
For configs, save data, and logs outside of mod management, use the File Manager (Files tab), since DayZ doesn't have a dedicated World or Properties tab. See Installing Mods on Your DayZ Server for the full Workshop mod walkthrough.
Next Steps
Head to Installing Mods on Your DayZ Server for the step-by-step Workshop walkthrough — the flow is identical to Arma 3, since DayZ shares its engine lineage. Also running an Arma 3 server? See Getting Started with Arma 3 Server Hosting. Back up your world before a big mod change with Managing Backups & Restoring Your Server.