update_activity_view_from_fragment.kt

/////////////////////////////////////
// APPROACH #1
/////////////////////////////////////
// In the Fragment
val textView: TextView = activity!!.findViewById(R.id.custom_toolbar_title) as TextView
viewModel.fragmentName.observe(viewLifecycleOwner, {
    textView.text = it
})

/////////////////////////////////////
// APPROACH #2
/////////////////////////////////////
// In the Activity
fun changeAppBarTitle(title: String) {
    binding.customToolbarTitle.text = title
}

// In the Fragment
viewModel.fragmentName.value?.let {
    (requireContext() as? MainActivity)?.changeAppBarTitle(it)
}