Yes, we suggest doing this if your professor continues to teach test material up until the day of the test. You want your classmates to have as much relevant information as possible.
You can upload multiple documents to any set of materials, so if you've uploaded a study guide for an exam, we suggest going back to that same study guide, then uploading additional documents to the ones that are already there.
Here's a quick guide you can look at:
If your classmates bought the materials before you added more info or edited your study material, they will not have to pay again to view the new documents.