Hi there First time posting on the forums. I'm currently running Ubuntu Mate 15.04 on my raspberry Pi 3 and I'm trying to upgrade to 16.04 Now I'm guessing I have to follow the upgrade tree, and upgrade to 15.10 first which the upgrade command did (I used sudo do-release-upgrade). All seems to be working fine, and activity monitor shows that I am now on 15.10 instead of .4; but the terminal has been sitting on 'setting up cups (2.1.0-ubuntu3)...' For over two hours.
The Raspberry Pi edition is a bit different to the desktop counterparts, it's not recommended to not upgrade across point releases (like 15.04 β 15.10).
Doing so will cause glitches later, and may lead to an unbootable system due to the kernel being left behind in the upgrade. (The kernel is updated separately using sudo rpi-update)
Some quotes from the project leader:
Please back up all your data, applications and then "flash" the new 16.04 image over the old one, which results in a nice, clean installation.