Seems like you’d have to jump through hoops to avoid updating and continuing to play it while connected to Steam normally. #1 you should go into the Properties of the game and set it to not automatically update. But that won’t fix your issue, as it will trigger the update when you attempt to launch it. Also set it to not allow updates while playing the game, just in case.
Some games you can go into the steam directory and launch the .exe yourself, and that might work. There’s also Steam Offline Mode, but you may want to set it up today before the update drops and test how it works (I’ve never used it). Maybe you’ll have luck disconnecting from the internet before launching the game after the update is available. If you’re not able to completely disconnect your machine for whatever reason, you can get a program like NetLimiter and block the steam applications from accessing the internet temporarily.