remove the third party repository you have added to to the sources.list file.
Open your sources.list file by typing:
sudo nano /etc/apt/sources.list
if it asks for password, enter it.
Scroll down to the repository that you want to remove.
Delete the line of code.
Save the file and exit.
Update your system:
sudo apt-get update
you may also want to upgrade your system by typing:
sudo apt-get upgrade