The first step is slightly different depending how you installed the server:
If you installed through steamcmd:
- Go to your steamcmd folder
- Open the steamapps folder
- Open the common folder
- Open the U3DS folder
If you installed through the Steam client:
- Right-click Unturned Dedicated Server in your Steam Library
- Select Properties...
- Hover over Manage
- Click Browse local files
There should be a Logs folder inside. The files are named according to the Server ID. This is the name from "+LanServer/ID" or "+InternetServer/ID". The most recent or currently running file is "Server_ID.log", and "Server_ID_Prev.log" is the log from before the server was restarted.