Can i change shutter speed on a video?

I have a Nikon D5100 and I want to shoot a video but I need it to be blurred at some points, (like when you take a picture with slow shutter speed and there are lights coming out blurry). I tried changing the shutter speed but there's no difference. Any help?

Blur is easiest done by defocus. Set the manual focus for the nearest possible position, and speed up the shutter to force the iris open.