How do I make my 70's schwinn cruiser lower?

The bike in the video has a customized fork. The frame has not been modified to make it lower, it was that small to start with. No way to tell, but the rider is probably a lot taller than you are. Smaller wheels is not the solution, that would make the saddle lower to the ground, but the pedals would also be lower making it likely they would hit the ground when you lean into a turn. Smaller wheels would also affect the way the bike handles. If your saddle is all the way down and you still find the bike is too big for you, the only way to fix things is to find a smaller frame.