I didn't find any question about this.
I have a computer with pre-installed Windows 8, UEFI interface. Is it possible, and if so, how to remove Windows 8 completely and just keep Ubuntu?
You would just need to remove the Windows 8 partition using disk usage analyzer or you could alternatively do a fresh install of ubuntu, while in the setup for ubuntu just select remove windows and use ubuntu (all depends if you mind losing data or not if you do the removing the partition is easier)
for more info look here Partition help