Start steam without updating
To do so, you'll need to switch your desired build to be the new 'default' build.If you created new depots in the process of adding this new content, make sure you add that new depot to all store and key packages. Once you're ready to release your update on Steam, you'll want to tell customers and provide a place for focused feedback.
For general information about communicating with customers, see our Moderation Best Practices documentation and watch Robin Walker’s full presentation from Steam Dev Days.
Updating and patching your game is one of the best ways to communicate with customers.
If you hear about customers experiencing issues with your game, there is no better response than to fix the issue as quickly as possible (if reasonable).
This also demonstrates to customers that you are listening and working to address the issues that are being identified.
Iterating and updating your product is essential, but your communication around the update can make a big difference.