It keep saying to change var to let on the FactBook.swift file

I'm going to take a guess here, but I agree with Fuad - more information would be helpful.

My guess is that Xcode is suggesting (or warning) that you should change a var to let. This means that you declared something whose value has not been subsequently changed (or mutated).

Here is an example with a function that takes a String value and prints a greeting if said value is not nil. If however the value is nil, then another message is printed.

func sayHello(name:String?){
    guard let name = name else {
        var nameNotProvided = "Name not provided"
        print(nameNotProvided)
        return
    }
    print("Hello \(name)")
}

In the above code, the "nameNotProvided" variable is not being mutated by anything and thus should be declared as

let nameNotProvided = "Name not provided"

Finally, even though this is being presented as a warning from the compiler and won't stop you from building and executing your app, you probably should take care of it. Using the let keyword when its appropriate will make your code easier to reason about and reduce the chances that something that should remain constant does not change unexpectedly. It also makes your intentions clear to other developers in the future.

Hope this helps.