The old ones had a split metal ring that held the assembly together. So you loosen the bolts underneath that hold the disposal snug, then you dig the metal ring out of all the corrosion and gunk with a screw driver and pry it off and then everything comes off. I'm sure there are YouTube videos that show this.

New ones are easier to put on. A lot of the new disposals are plastic. I recommend paying a little more for one that is all metal. Unless you don't think you are going to be in the house long. Then by all means go cheap so the next guy has to deal with the fun.