How do I change the startup screen of my unity game?

I’m guessing that you want to load straight into the game without displaying the screen resolution dialog?

You can change this behaviour by setting “display resolution dialog” to disabled in your project’s player settings.

Related
https://forum.unity.com/threads/unity-screen-mode-framework-for-windows-how-to-sobstitute-the-missin-old-resolution-dialog.947598/

There is an example project here: DesktopSamples/ScreenSelectorExample at master · Unity-Technologies/DesktopSamples · GitHub