There is no official support for changing a splash screen, although there is a feature request on Right-CLick Select.
You can't simply replace the splash screen but you can create an Application Template that has a custom splash screen. These steps are taken from this tutorial on YouTube:
Create a new folder in PATH_TO_BLENDER_INSTALLATION/version/scripts/startup/bl_app_template_system
copy your startup files from your startup area (on Windows AppData/Roaming/Blender Foundation/Blender/VERSION/config; someone else will have to verify OSX) to the folder you just created.
add the two png files that you want for your splash screen (splash.png and splash_2x.png) to the template folder you created in the first step.
setup a shortcut (windows) or alias (your favorite shell) to start blender using the application template you just created. The command is blender --log --app-template TEMPLATE_NAME
Anytime you use the shortcut (or alias) you'll have a setup with your new splash screen.
NOTE: "splash.png" must be spelled that way and must be a 1000x500 png file.