Answer

Using the ZKS Toolkit for Salesforce its easy the update the contact with the following code snippet.

theRecordId - Primary key for the Cotnact Record

ZKSObject *contact = [ZKSObject withTypeAndId:@"Contact" sfId:@theRecordId];
 [contact setFieldValue:@"John" field:@"LastName"];
 [contact setFieldValue:@"Smith" field:@"FirstName"];

 NSArray *results = [sforce update:[NSArray arrayWithObject:contact]];
 ZKSaveResult *sr = [results objectAtIndex:0];

 if ([sr success])
    NSLog(@"updated contact id %@", [sr id]);
 else     NSLog(@"error updating contact %@ %@", [sr statusCode], [sr message]);