How do I repond to a summons to modify child support without a lawyer?

Check the summons for a day to appear in court. Remember the adage: a lawyer who represents himself has a fool for a client. I have attached a web address that should give you some help: Modification of Child Support - Findlaw for the Public -

You need to make copies of your tax returns, current pay stub and show proof of your expenses (rent receipts, mortgage statement, checking account stmt, etc.) then take them into court with you.

Mostly it depends on what the summons says and what you have or don't have going currently. Your question is too far open to give really good answers to. Also if you are the custodial parent then you may get free legal help and in some places the noncostodial parent can get help also. What is going on in your situation ? Let us know.