How do I change the difficulty from the map voting screen?

I've seen this done but have no clue how to do it.

In game after hitting Esc>Map Voting, there was an option to change game type with options Survival/Objective. There was another option for difficulty Beginner, Normal, Hard, Suicidal, Hell on Earth. And another option for game length short, medium, long.

I was told this was done using the game configuration option under the "Server" tab yet I have no clue how to use it.

Does anyone have a guide? Or can anyone explain?

How do I change the difficulty from the map voting screen?