How do I change the text of one of the columns of a specific item if I
know the index I want to change at?

mylistview.Items[index].Tag
returns a null reference exception if I attempt to use it.

I am displaying data that changes in real time and needs to have
certain columns update when a separate thread invokes an
UpdateMyColumn(int index, string newText) method on the form.

Should I be using a different control, instead?

listView.Items[index].SubItems[index1].Text = "your text";