How to change single user modle in Windows server 2012/R2
1. RDP into server and Run gpedit.msc by powershell or CMD
2. Go to Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Connections
2.1 Set Restrict Remote Desktop Services user to a single Remote Desktop Services session to Enabled.
2.2 Double click Limit number of connections and set the RD Maximum Connections allowed to 1.
3. Reboot server.