Graduate student salary increase only for new students
Assuming the poster is in the United States:


You can request a salary increase.  Sometimes requests are granted.
You can leave the university to get higher pay elsewhere.
You can organize students in to a union, which will strike if an acceptable salary is not provided by the university.  Most universities in the United States do not have a graduate student union which negotiates an employment contract.  These are different from the student union which offers campus recreation at universities in the United States.


On your own, a PhD student has very little bargaining power.