How to call a function of a vc inside another vc.

main.swift

//Inside the main vc i put a function like this
    //Function fetches data and calcs the total
    func changeNavTitle() {
            //[...]
            // It does some stuff and then updates navigation label (you can change this to another label)
            titleNav.title = "\(sum)\(currencySymbol)"
            fetchedResultsController.changedContent = false
    }
    
//Inside the other vc I call the function like this:
//I have an IBAction that's calling the dismiss
//If your dismiss is handled otherwise and you can't put both in one function, you can still add the code in the
//completion of dismiss
//If you need help with the completion, watch this treehouse course: https://teamtreehouse.com/library/closures-in-swift
@IBAction func save(_ sender: Any) {
        
        //[...] does some stuff for CoreData

        //Calls the function to update the total
        //IMPORTANT: Change the "TableViewController" to the class name of your vc. Else it wont be able to access that function!!!
        let presentedBy = presentingViewController as? TableViewController
        presentedBy?.changeNavTitle()
 
        
        dismiss(animated: true, completion: nil)
    }