The pi has been renamed correctly but your Windows machine still thinks it is called "DEBIAN". The Pis IP address is still being resolved to "DEBIAN" either by your Windows machine's local host file or (more likely) a naming service on your network, such as DHCP on your router. You could try waitin...

I followed all instructions I could find to change the hostname on a Pi running Jessie.
It looks as if all is good on the Pi's side:


But doing a "ping -a xxx.xxx.xxx.xxx" returns "DEBIAN" as the hostname:


Any ideas?