Once you have the game installed on your machine, and added to Galaxy, select it from the side-bar on the left. Then click on MORE -> Settings, and look for the BETA CHANNELS setting in the top-right corner. To enable beta branches change this setting to ON.
Now, if the game supports beta branches, you can choose the one you want to use from the UPDATES: CHANNEL drop-down list.
If it is a private beta branch select ADD PRIVATE CHANNELS from the bottom of the list, input the password for beta branch, confirm by clicking SUBMIT, open the drop-down list again, and select the beta branch you want to use.