It depends on the food you're looking up.
Would you be able to give an example?
I've only come across a handful of food items that say "per serving" and won't let me change it. They're usually Brand Named foods in the system.
The best thing to do, if possible, is to use the information on the package. Or you can find nutritional information online and then plug that information into the online calculator to help you get the correct number of Points.
~Made of star stuff~