Editing XML is a little difficult in iOS. You need to parse the original xml to a model and then form the xml.
You can make use of 3rd party library such as GDataXML for forming XML from a data source.
//Edited user info saved in a dictionary
NSDictionary *dictionary = @{@"Name": @"Jim", @"Option":@"1"};
GDataXMLElement *rootElement = [GDataXMLElement elementWithName:@"Dat"];
[dictionary enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
GDataXMLElement *element = [GDataXMLElement elementWithName:key stringValue:obj];
[rootElement addChild:element];
}];
//xml document is formed
GDataXMLDocument *document = [[GDataXMLDocument alloc]
initWithRootElement:rootElement];
NSData *xmlData = document.XMLData;
NSString *filePath = [self savedXMLPath];
//XML Data is written back to a filePath
[xmlData writeToFile:filePath atomically:YES];