Upgrade virtualbox to 5.0.16

I have virtual box 5.0.14 on my Ubuntu with extension Pack 5.0.16 which makes virtual box stop working. how do I upgrade my VB to latest version? I followed instruction in the screen shot but there are some errors. when I add Virtual box repository via the second command nothing just happens. then I run the Oracle Public Key downloading command (third one in the picture) it returns this: no valid OpenPGP data found. I run "sudo apt-get install dkms" and it says dkms is already the newest version.
what am I missing here?

Actually I want to upgrade VB rather downgrading the Extension pack. I looked at the official website, it says I have to add a source to my source list but when I add the line in terminal I get this.

I even tried to add it form the software center but it didn't work. obviously there is a way to install the latest version, isn't it? why is this getting so mixed up?!!