For this guide you will need:

• a copy of a .mp4-file of your choice that is named "Master_Intro" (without the ")

Pretty simple:
1. Go to your Steam library
2. Right-click "Infectonator: Survivors"
3. Click on properties
4. Go to the local files section
5. There should be a button called "browse". Click it. Your file manager should now open up in a new and separate window.
6. You should be able to see multiple files now. Go into the "assets" folder.
7. Now go into the "videos" folder.
8. There you see one .mp4-file called "Master_Intro.mp4". Delete it and put your replacement file in.

That's it! You can now start the game to marvel at the result

Even easier and similar steps:
1. Go to your Steam library
2. Right-click "Infectonator: Survivors"
3. Click on properties
4. Go to the local files section
5. Click on "Verify integrity of game files"

That's it, again! Steam will automatically download and replace the file you placed in the folder.