Assuming you have already made the 'bootable win7 usb flash drive'...
While the laptop is powering on, look for the message at the bottom of the screen during the bios startup phase, that reads something like 'Press F-- for Boot Options' and press that F-key.
From the list presented locate the flash drive and select it. You may have to use the arrow keys to navigate to it and press enter while its highlighted.
If your system is a uefi/efi system look for the flash drive with a "UEFI:" in front of it. It may even say UEFI: Windows 7 Installer or something like that.