you posted in the wrong forum

If you mean the Steam Client Beta, you go Steam, Settings, Interface
And you should see an option there to get into the beta

If this is for something else you will need to say what at least, or post in the Hub of what ever game or software that is

Originally posted by XEpleslang-:

How to get access to the beta?

As the title says: how can I get to test the beta version? I saw one clip about a new and big update

The client beta doesn't have anything big and special that the mainline branch doesn't already have.

Right click that game properties> opt in to Beta.
Did they provide a code?