How to change the quality of the game while playing it?

I've created a dropdown and a list of quality options for example: 'Very high, high, medium, low' in the dropdown, however the quality doesn't change. I don't this code is right but if it's not, then send me the right one please.

The code:

 public void Quality_V_high ()
 {
     QualitySettings.SetQualityLevel  (6, true);
 }

 public void Quality_high ()
 {
     QualitySettings.SetQualityLevel (5, true);
 }

 public void Quality_medium ()
 {
     QualitySettings.SetQualityLevel (4, true);
 }

 public void Quality_Low ()
 {
     QualitySettings.SetQualityLevel (3, true);
 }

}

Here is a picture of the dropdown

And here is what it looks like in the inspector:

It will really help me if someone answers this because i need to move on in the game. Please also note that the code is written in c#.