If update-notifier is giving you the "do not update" message you need to look at the file ~/.update-notifier/apt/apt-get.log to find the reason for the message (what packages are being held). You can see the contents of that file by opening a terminal and entering:
cat ~/.update-notifier/apt/apt-get.log
Without that information it is impossible know exactly what advice to give, we can only guess and it is guessing that can lead to difficulties. If you don't understand what the log file is telling you then post it here and ask for advice.
Terry