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There are different ways for doing that. You can take reference to www.osdev.org for details.

The easy way for me was going with linux and extracting MBR for drives:

1. Backup first

dd if=/dev/sda of=~/[original_bootloader.bin] bs=512 count=1

2. Disassemble the bootloader

ndisasm -b16 -o7C00h ~/[original_bootloader.bin] > ~/[original_bootloader.asm]

3. Make your modifications and reassemble

nasm ~/[original_bootloader.asm] -f bin ~/[modified_bootloader.bin]

4. Overwrite the bootloader

dd if=~/[modified_bootloader.bin] of=/dev/sda bs=512 count=1

‘sda’ is your default primary (bootable) drive. You can modify the .bin file and test in your development enviroment and deploy the successfully edited bootloader  to system.