No, you may not extend your stay on C2 visa. If you want to extend your stay you should apply for appropriate B-1 or B-2 visa, or can travel on Visa Waiver Program.
Note: If you seek layover privileges for purposes other than for transit through the U.S. (for example, to visit friends or engage in sightseeing), you will have to qualify for the type of visa required for that purpose.
No, you may not change your status while on C-2 visa.
If you leave the U.S. and want to re-enter the U.S., you have to reapply for C-2 transit visa. You cannot apply for a change of status.