I guess this needs a little more explaination...
- Open Steam Librabry
- Right Click Factorio -> properties
- Register "Beta"
- In the drop down use:
"1.1.78 - Factorio Experimental 1.1.78"
OR
"1.1.x - Latest 1.1 Experimental (1.1.78)"
It think its obvious, in both cases you will get 1.1.78 but if you use the second option "1.1.x - Latest 1.1 Experimental (1.1.78)" the base game gets always automatically updated to the latest experimental branch.