This link might help:
Be sure to back up any crucial data you have on Windows to external media before you do this, because your drive will be repartitioned and all data will be lost.
Short version for what you want to do:
Download the Ubuntu installation *.iso
and burn it to disc, being sure to use the "burn image" tool in your burning program. If you can't use a disc, you can use unetbootin
to make a bootable USB stick.
Turn off secure boot in the UEFI boot settings.
Boot to the Ubuntu install media. (You may have to check the "boot order" settings in your UEFI/BIOS.)
Install Ubuntu and follow the steps. When you are asked, tell it to replace
the existing operating system.