You need to use some type of video editing software like adobe after effects or premier pro. I’m not sure if this file uses a Luma matte or an alpha channel but if you open the _ALPHA file and you see the donuts just with a black background all you need to do is add that to your software then place your new background below that layer. If it is a luma matte you’ll know this because the _ALPHA file is all black and white. Then you would just need to make a luma matte of the original file then again just place your new background on the layer below it.