You can contact your loan servicer who will advise you as to how to make payments on your loan.
How do I go about changing my repayment plan if my financial circumstances change?
It is best to contact your loan servicer about changing your repayment plan. However, you might want to use the Loan Simulator to get a look at which plans you may be eligible for and see estimates for how much  you would pay monthly before contacting your loan servicer.
I have not been able to make a couple of loan payments in the last months and am wondering what I should do about it?