The basics
What is Form I-539
Form I-539, "Application to Extend/Change Nonimmigrant Status", is how you ask USCIS to either keep your current status longer (an extension, e.g. more time as a B-2 visitor) or switch to a different status (a change, e.g. B-2 to F-1 student). If family members are extending with you, each one is added on a Form I-539A supplement — one short page per person. This app fills the I-539 for you from your answers and documents; you review every value before it is printed.
What if…
Do I sign the I-539 myself?
Yes — every applicant aged 14 or older signs their own form with a pen, in black ink, on the printed page. A child under 14 does not sign; a parent or legal guardian signs for them. Digital or typed signatures are not accepted.
Official sources
This is general information, not legal advice.