Once a patent application is filed with the United States Patent and Trademark Office, it is a recognition of what the inventor has at the time of filing. Invariably, the application will get rejected within the 1st two years. After a rejection, an inventor does have an opportunity to make modifications to the application. However, there are important restrictions to making any amendments. One is the prohibition against adding new stuff to your invention.
Making a change after you as the inventor have revealed what you know to the patent office could change the scope of what you said was your invention. Any change to your previously filed patent application could expand upon the contents you originally disclosed at the time of filing. This upsets the assumption of the examination process, i.e. that you fully disclosed your invention at the time you filed your application. Thus, the patent laws prevent an inventor from amending a patent application after filing that would add new matter to that application. 35 U.S.C § 132 ("No amendment shall introduce new matter into the disclosure of the invention.")
This problem is based on the idea that when an inventor makes an application for patent that application is considered a statement by the inventor that the invention itself "was made at the time the application was filed" Automatic Weighing Mach. Co. v. Pneumatic Scale Corp., 166 F. 288, 293 (1st Cir. 1909). Adding new matter means that the inventor could not have completely disclosed the invention when the application was filed, which would have made the examiner's work on that application a waste of time and effort.
So, yes an inventor can change the patent application after it has been filed but cannot add new matter in so doing.