Try this code, it should work:
NSString *yourStringDate = @"Saturday_June 22_2013 11:44 AM";
NSDateFormatter *dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
[dateFormatter setDateFormat:@"EEE_MMM dd_yyyy hh:mm a"];
NSDate *date = [dateFormatter dateFromString: yourStringDate];
//Set the required date format
[dateFormatter setDateFormat:@"dd-MM-yyyy hh:mm"];
NSString *convertedString = [dateFormatter stringFromDate:date];
NSLog(@"Converted String : %@",convertedString);