Modification of child support


I know that either party can request modification if the support order is more than 3 yrs old and if they believe the amount would change by 15% or more. But how about if the order is not that old?
What if circumstances have change…less income, child aging out, no day care expenses, increased health insurance costs…what do you have to show and do to get your support modified. Does it still have to be 15% or more change?