First, you have to get the Bitlocker Key for that drive (click backup bitlocker key from the bitlocker menu right below where you turn it off in Windows, then print it to a pdf)
Boot to Windows Recovery menu (hold shift while clicking restart)
Then click the troubleshoot option then the option to go to a command prompt
it will prompt you for that numeric bitlocker key
enter that and it will give you a command prompt from that recovery drive
then you can just enter: manage-bde –off C:
it will say “decrypting drive…”