Non business visitor but getting B1 stamp?

http://www.uscis.gov/files/form/i-539.pdf
Link above is to the Petition to Change/Extend Non-Immigrant Status.
http://www.uscis.gov/files/form/i-539instr.pdf
Link above this line are the detailed instructions including fees and where to send application and supporting evidence.

USCIS guidelines for doing so as a B-2 (You may wish to enter: B-1/B-2 non-business visitor for pleasure (Yes, that's what they call it rather than leisure), quite a few visas are marked B1/B2.
Here are the USCIS TIPS for filing this type of extension of stay request:
Department of Homeland Security
U.S. Citizenship and Immigration Services
Form M-752, Helpful Filing Tips for Form I-539
Tips for Filing Form I-539, Application to Extend/Change Nonimmigrant Status, for an Extension of Stay as a B-2 Nonimmigrant Visitor
When filing Form I-539 for an extension of stay as a B-2 nonimmigrant visitor, it is important to fill out the application completely and provide all required information with your initial filing. You must:
A. Fill out all required fields and sign the application; B. Submit a photocopy of a valid passport and an original Form I-94, Arrival-Departure
Document, for you and each family member included in the application;
C. Submit a description of your means of financial support while visiting the United States (include any relevant documentary evidence, such as bank or financial records);
D. Submit a written statement that explains the following: 1. The reason for your request to extend your stay;
2. Why the extended stay would be temporary, including what arrangements you have made to depart from the United States (include any applicable evidence, such as a copy of a return plane ticket); and
3. Any effect the extended stay may have on your foreign employment or residency. For example, you may want to explain how you can be excused from your employment for such an extended period of time.
Note that the items listed above are required when requesting a B-2 extension of stay on Form I-539. Failure to provide this information with your initial filing may result in your case being denied or delayed. There are no appeal rights for applications for extension of B-2 nonimmigrant stay; therefore, it is important for you to fill out the application completely and provide all required information.