I'm buying a new PC, and would like to change Windows with Ubuntu, planning to later get a refund of the licence key (€60 or so). I'm new to this UEFI issue, and wondering how I'll be able to disable it without going through Windows (thus activating the licence and voiding any claim of refund). Anybody could help with it?
The short answer is "it depends."
The long answer is that different computers come configured in different ways. If your computer enables you to select an external boot medium (probably a USB flash drive on a modern computer) using the firmware's built-in boot manager from the start, you should be able to boot an Ubuntu installation medium and replace Windows without activating it. OTOH, some computers come configured in a way that makes this difficult -- you must normally boot Windows in order to enter the firmware setup utility to change the boot options to access the firmware's built-in boot manager. AFAIK, the only workaround if you've got such a computer is to unplug the hard disk, which will normally cause the firmware setup utility to launch when you boot. This may be an acceptable solution in some cases, but for some computers, unplugging the hard disk may require opening the computer in such a way that the warranty may be voided.
You should be able to boot Ubuntu from flash or DVD as a live Distro regardless. Ubuntu is pretty UEFI aware. Older hands would then image the disk to an external dive as a backup. It should be pretty empty so it will compress well. If you cannot boot the flash drive (I have this with Kaspersky a lot), then there should be a hot key such as F2, F10, F12 for entering setup. Unless this is password protected you can then toggle secure boot. Then install Ubuntu over the top of the disk. The best way is to replace the disk, and keep the original for possible re-sale. The only advantage in disabling secure boot really is that you do not end up with a small 500MB partition for bootloading. Toggling secure boot is a royal pain on most all-in-one PCs and convertible tablets.