I have test CO with integer field "value". At first I set value to 20.
NSMutableDictionary *updateRequest = [NSMutableDictionary new];
[dictionary setObject:@(1500) forKey:@"inc[value]"];
[QBCustomObjects updateObject:obj specialUpdateOperators:updateOperators delegate:self];
Now I have object with value = 1520.
Be careful with permissions. If you don't right setup it, you'll get "Wrong permission" error