How to append array in file - objective-c

Question

I am trying to write main array in a file and this main array contain sub array . Now whenever I write main array the sub array get overwritten .
Below is my code.
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
#autoreleasepool {
char fname[10];
printf("Type your first name:\n");
scanf("%s",fname);
NSString *firstname = [[NSString alloc] init];
firstname = [NSString stringWithUTF8String:fname];
char lname[10];
printf("Type your last name:\n");
scanf("%s",lname);
NSString *lastname = [[NSString alloc] init];
lastname = [NSString stringWithUTF8String:lname];
long mnumber = 0;
printf("Enter your register mobile number:\n");
scanf("%ld",&mnumber);
NSNumber *mobilenumber = [NSNumber numberWithLong:mnumber];
long anumber = 0;
printf("Enter your account number:\n");
scanf("%ld",&anumber);
NSNumber *accountnumber = [NSNumber numberWithLong:anumber];
NSURL *docdir = [[NSFileManager defaultManager] URLForDirectory:NSDesktopDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:NO error:nil];
NSURL *filepath = [docdir URLByAppendingPathComponent:#"Dexter_apps/filetesting/data.plist"];
NSArray *data = #[firstname,lastname,mobilenumber,accountnumber];
NSMutableArray *mainarray = [[NSMutableArray alloc]init];
mainarray = #[data];
[mainarray writeToURL:filepath atomically:NO];
}
return 0;
}