You can not write in to main bundle. You only can read from main bundle. If you want to write an file you need to place it in to the documents directory of your app.
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *path = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.plist",plistName]];
If you need the plist from the main bundle you can copy it first in to the documents directory then modify it. It is advised to have a check to ensure it is copied only once.
NSError *error;
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *path = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.plist",plistName]];
NSFileManager *fileManager = [NSFileManager defaultManager];
if (![fileManager fileExistsAtPath: path]){
NSLog(@"File don't exists at path %@", path);
NSString *plistPathBundle = [[NSBundle mainBundle] pathForResource:plistName ofType:@"plist"];
[fileManager copyItemAtPath:plistPathBundle toPath: path error:&error];
}else{
NSLog(@"File exists at path:%@", path);
}