Restart your computer, enter the BIOS/UEFI (usually F2, Del, or F12), and change the boot order to boot from USB/DVD first.
Think of an ISO file as a digital snapshot of an optical disc (like a CD or DVD). But instead of holding music or a movie, the Ubuntu ISO holds a complete, bootable file system. It contains millions of files—kernels, drivers, desktop environments, and installers—all compressed into a single .iso file. unbuntu iso
