You owe the IRS money and you're paying an installment agreement but your situation has changed. You can't afford to keep making that payment. The first thing you need to do is let them know that. You can contact them either by phone or by mail or probably the best if your situation justifies it through a tax attorney or a CPA who's representing you.

What you'll want to do is have the documentation to show the IRS why your situation has changed and why you can't afford to continue making the payment. If you do that and you have the documentation to support that and you fill out the forms that apply to 433A, 433B or the 433F, then there's a very good chance you can have your installment agreement amount changed. You definitely don't want to just stop paying or change the amount without dealing with the IRS and letting them know about you're changing circumstances.