Payment Coupons vs Actual Income

What happens if you make way less the next year and you cannot afford to make the quarterly payment and in fact that amount would be an over payment because you are making less?  Can you just pay less or do you have to call the IRS and get new coupons or what?

 

On the flip side what if you make way more?  Are you required to add to your quarterly payments and if so how do you know how much more to add?

 

Thanks!

 

Kelly