Cannot upgrade to 16.04

I'm trying to upgrade from Kubuntu 14.04 to 16.04 and having a great deal of trouble about it.

I became aware that a new version is available via popup notification on the desktop asking me if I want to upgrade to 16.04. I tell it "yes, update." It goes through a series of processes and ultimately gives me an error message saying I don't have enough space in some part of the hard drive, and to empty my trash and clean-out temporary files from old distros with

sudo apt-get clean

I tried to burn an iso image of the new distro onto a DVD-R, thinking that it might be simpler to use this method and just wipe-out everything (including the clutter apparently keeping me from having enough disc space to do this the standard way) and just starting from scratch with 16.04.

I completed the boot disc, then I restarted the computer, but it didn't boot from the disc. My hard drive is encrypted and has been from setup, if that makes a difference in the ability to boot from disc. In any case, neither restarting the computer nor shutting it down completely, waiting 30 seconds to make sure and then turning it back on gets it to boot from the disc. I always just get asked for my encryption password, and when I've provided it, it takes me to the standard user-login form.

I don't know how many other options I've got for how to make this happen, and it seems that what's required in order to clear-out enough space in the specific part of my hard drive that's needed for this is too technical for my level of programming knowledge (which honestly, is next to none.) Does anyone have any ideas?