To return to previous windows version from Win 10:-
- Click 'Windows Start' icon
- Select 'settings'
- Select 'Update & Security'
- Select 'Recovery'
- You should see an option on the right of the 'Update & Security' panel to go back to your previous Windows version.
Windows 10 no longer supports Windows Media Player but it was rubbish anyway. A far better solution (both pre-W10 computers and with W10) is to use VLC:
http://www.videolan.org/vlc/download-windows.en_GB.html
(Use 'Installer Package' if you've got a 32-bit version of Windows. Use '64 Bits Installer' if you've got a 64-bit version).