"Boot"? Boot what? A tablet? A PC? Other? Boot into what? Some OS? (Which one?) And "USB"? What kind of USB device? What's on the device?
In general, you power off whatever device you want to 'boot', connect whatever 'USB' you want to boot from, and then power on the device you want to boot.
That's "How". But it's potentially possible that many steps are needed to get there, depending on what all of the equipment is, how it's all been configured and what the actual problem is that needs to be solved.
Any number of problems might not be appropriate to booting with USB, e.g., various BIOS access issues.
We need a lot more info like Raven mentioned. I have not looked lately but the BIOS determines the boot order of devices. Not sure if USB is before HD or even the DVD/CD drives.
As the tag mentioned "windows os", you have to download windows usb/dvd download tool to overcome your issue. Rest is the here: http://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
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In general, you power off whatever device you want to 'boot', connect whatever 'USB' you want to boot from, and then power on the device you want to boot.
That's "How". But it's potentially possible that many steps are needed to get there, depending on what all of the equipment is, how it's all been configured and what the actual problem is that needs to be solved.
Any number of problems might not be appropriate to booting with USB, e.g., various BIOS access issues.
http://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool