@Vivaldi Easiest install I have done in ages actually.

I plugged in an Ubuntu USB bootdisk and held the "Option key" on startup. From here I selected the Ubuntu boot disk. Ubuntu fired up and gave an option to install I clicked next a handful of times and then entered stuff like a new username and password, location and keyboard layout.

On reboot, I went to bluetooth settings and added the mouse (I used a spare wired mouse up to that point).

Finally I logged in and opened the Ubuntu software store. We are still a featured snap (as you can see in image). I clicked to install that, entered password again. Done!

Obviously more work if you need to transfer data (which I did not). Otherwise, very, very easy.