how can set boot sequence 

About Boot Priority
To boot from a CD, DVD or USB device, make sure that the device has boot sequence priority over the hard drive.
BIOS Boot
BIOS (Basic Input Output Subsystem) is a programmable chip that controls how information is passed to various devices in the computer system. A typical method to access the BIOS settings screen is to press ESC, F1, F2, F8 or F10 during the boot sequence.
BIOS settings allow you to run a boot sequence from a floppy drive, a hard drive, a CD-ROM drive or an external device. You may configure the order that your computer searches these physical devices for the boot sequence.
The first device in the order list has the first boot priority. For example, to boot from a CD-ROM drive instead of a hard drive, place the CD-ROM drive ahead of the hard drive in priority.
Before you set boot priority for a USB device, plug the device into a USB port.
To specify the boot sequence:
Start the computer and press ESC, F1, F2, F8 or F10 during the initial startup screen. Depending on the BIOS manufacturer, a menu may appear.
Choose to enter BIOS setup. The BIOS setup utility page appears.
Use the arrow keys to select the BOOT tab. System devices appear in order of priority.
To give a CD or DVD drive boot sequence priority over the hard drive, move it to the first position in the list.
To give a USB device boot sequence priority over the hard drive, do the following:
Move the hard drive device to the top of the boot sequence list.
Expand the hard drive device to display all hard drives.
Move the USB device to the top of the list of hard drives.
Save and exit the BIOS setup utility.
The computer will restart with the changed settings.
Some computer manufacturers allow you to select the device that contains the boot sequence from a special device selection menu. The example below uses a Dell system board.
To set boot priority using a device selection menu:
When the computer starts to boot up, after the manufacturer's ID screen, press F12 several times. The device selection menu appears.
Use the up and down arrows to select CD-ROM or an USB Flash Drive.
To boot from the selected device, press ENTER.