Right click DayZ in your Steam Library
Click Properties
Click the Betas tab
In the drop down menu choose...
experimental =  preview testing version.
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To revert back to Stable Branch.
 
Right click DayZ in your Steam Library
Click Properties
Click the Betas tab
In the drop down menu choose...
NONE = opt out of all beta programs
Close properties.
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NOTE : No code is needed. DayZ needs to be closed when changing this setting. You will need to download files when switching between versions, size may vary.
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How to find Experimental Branch Servers...
 
Click the Change Server Tab on welcome page. Click version tab or...
Scroll down list of Servers until you find a Server with a Green Tab.
Click on Server to Highlight.
Click on Host Tab (top left browser option).
This will group all Experimental Branch Servers.
 
Select Server of your choice. ************************************   NOTE : Separate Character for each Game Version (Experimental / Stable). ************************************** A useful Topic... http://forums.dayzgame.com/index.php?/topic/161796-howto-test-experimenal-builds-help-the-dev´s-to-improve-quality/#entry1629015