I have string from web service like 12/31/2013 09:12:15 A.M.
Now I Want convert it like 12 Dec 2013 09:12:15 A.M.
with the use of NSDAteFormatter
I'm giving you the answer, but downvoting your question for being too elementary and whose solution could easily found with a simple search.
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"MM/dd/yyyy hh:mm:ss a"];
NSDate *date = [formatter dateFromString:myDateString];
[formatter setDateFormat:@"dd MMMM yyyy aa:mm:ss a"];
NSString *newDate = [formatter stringFromDate:date];