I want to switch from Windows 10 to Ubuntu but the problem is that USB won't boot up, it just skips and Windows 10 boots up. I tried USB on second PC where I have Ubuntu and it boots fine from USB. Any idea what can be wrong ?
Boot your first PC into the BIOS and look for the boot order settings.
If your Ubuntu set up is on a USB drive and it works on one PC and not another, then the problem may be the BIOS setting of the first PC. The Boot device order is set in the BIOS and frequently (but not always) defaults to looking at the Hard Drive first. You would need to change the setting to look at the USB drive first. Your second PC may already have its BIOS set to check for a boot block on USB before it checks on the Hard Drive.