Ubuntu - How to fix Failed to fetch...404 Not Found when apt-get update

i get this error when trying to update with sudo apt-get update : E: Failed to fetch http://ppa.launchpad.net/vokoscreen-dev/vokoscreen/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found W: Some index files failed to download. They have been ignored, or old ones used instead. That problem caused by your bad repository. To Fix it, just open system settings (on top right your ubuntu) - Software

• #متفرقه
• #apt
• #get_update
• #ubuntu
• #linux
• #failed