Follow these steps:
Ensure the phone is turned off.Press and hold the VOLUME DOWN button.Press the POWER button.Continue to hold VOLUME DOWN until the Download mode is displayed.Press VOLUME DOWN to go to Reboot to bootloader.Press the POWER button.

How do I enter bootloader mode?

HTC Devices: Press and hold the volume down button, then power up the device while keeping the volume down key pressed. The bootloader on HTC devices is referred to as “fastboot mode.” Motorola Devices: Press and hold the volume down and power buttons for a few seconds.

How do I get to the boot menu on Android?

How To Access Android Recovery Mode
Turn off the phone (hold power button and select “Power Off” from the menu)Now, press and hold Power+Home+Volume Up buttons..Keep holding until device logo shows up and phone restarts again, you should enter recovery mode.

The boot-loader code is stored within the /boot partition. Majority of Android handsets uses the Qualcomm’s Boot Loader, called LK (Little Kernel), barring Samsung, which uses their own form of boot-code.