Before editing, export sector 0 to a file. If you break something, boot from a live USB, open BootICE, and import the saved sector back.
The MBR is the first sector of your disk. If it's corrupted, your OS won't load. bootice manual better
: Backup, restore, or install Master Boot Records (MBR) and Partition Boot Records (PBR) for local or removable disks. BCD Editing Before editing, export sector 0 to a file
: On modern systems, it can manage UEFI boot entries, allowing users to reorder or delete firmware-level boot options directly from the Windows environment. 4. Advanced Sector Editing boot from a live USB