Hi there. Welcome along!
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 → 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. 