You may mail I-129 to extend your mother's B-2 from within US. I

You may mail I-129 to extend your mother's B-2 from within US. It takes several months for USCIS to process your I-129. As long as you have the receipt of your I-129 extension application from USCIS, your mother's status in US is valid. You don't need to care whether your I-129 extension application is approved or not.