B1/B2 Visa Extension requirements

[1blue1]

My parents are staying with me( 6 months as per entry visa), they have 1 year multi B1/B2 visa .
6-8 weeks before expiration they would like to apply for an extension ,possibly for another 6 months or what ever time frame they could get. Could you please tell me :
1) what shell we send as a "proof of financial support" and
2) regarding air ticket-shell we send a copy of the actual air ticket (6 months valid) or shell we extend ticket first and then send a copy of a new air ticket with extended dates for another 6 months?
Also I would like to know if we can send an Application at any USCIS center?
Thank you for your response!

[peace]

Advice; If they apply for an extension on a one year visa, chances of getting another visa after this visa expires will decrease. The officer
at the embassy will think of them as potential immigrants as they stayed so long on their first visa.