The easiest way to edit the boot manager of your operating system is to use a compatible tool. For this type of job I would like to recommend you use an application which manages your boot sector and an the same time keeps a backup of the sensitive data that is stored in that respective area.
EasyBCD allows you to modify everything that is related to Windows 7 or Windows 8 boot sectors. You can add another operating system, repair the MBR area or simply perform backup or restore operations.