In most cases these days, the threshold is an intrigal part of the door frame. it can really be dificult to remove and replace.
Here is what I would do: Get a sawzall and cut in the middle of the threshold all the way through. Once through you can pry the threshold out of the area. Then you can put the two pieces together to make a template for the new one.
This is how I do it one very stubborn ones. Most will come out without doing this method.