Your documents
Showing your ties to your home country
A visitor extension rests on the idea that your stay is temporary and you will go home. The clearest single document is a recent proof of your home-country address in your name — a utility bill, bank letter or lease. If the bill is in a family member’s name, a short signed letter from them confirming you live there works too. Other helpful ties: a job you are returning to, immediate family at home, property or studies. The assistant collects your foreign address and the proof for the I-539.
Official sources
This is general information, not legal advice.