There may be issues that change what I am about to say, as no question exists in a vacuum. so a consultation with an attorney is best.
This is general information only.
File Form I-539 with the fiilng fee ($300) and documentation of your reason for extending your visitor status. The maximum you can ask for is 6 month more months. Take a look at your current I-94 card and see is there is any restriction written on the back.
Show proof that you continue to have funds to stay here without being a public charge.
Show that you have a return ticket.
If the extension is for medical reasons, show you are paying for the treatment.
File before your current status expires
Consult an attorney if you have any doubts about how to do this.
Form I-539 is the form to change or extend a non-immigrant visa. The California Service center is estimating 2.5 months on these applications.
Contact me for further information if needed. 650-212-6781