Open a Terminal and type
sudo apt-get upgrade. Check if you see an error similar to:
E: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a'
to correct the problem.
It means that the upgrade was halted midway. You need to complete the upgrade manually by typing the following commands:
sudo dpkg --configure -a
sudo apt-get -f install
sudo apt-get update && sudo apt-get upgrade
If the error is something like:
E: Could not get lock /var/lib/dpkg/lock - open (11 Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/) is another process using it?
In this case, the upgrade process was unable to remove its lock file before exiting. Let's remove the lock files and try again:
sudo rm /var/lib/apt/lists/lock
sudo rm /var/cache/apt/archives/lock
sudo apt-get update
sudo apt-get upgrade
That's it. You should be good to go now.