Developer mode?

So I just found out that you can play Germany 1 (dev) still in a developer mode. I was wondering how I switch to that! Sorry i am new to rust, but I did a search and didn’t see anything on the forums… Thanks in advance!

[editline]18th December 2013[/editline]

I activated beta in steam which I was told to do, and I was told Rust would update but it didn’t. If that helps? Somebody please help me out here!