I have a list and inside this list I have a text box.
Since it's a list, the text box value changes for each row in the dataset.
OUTSIDE this list, I have another text box which is part of the HEADER.
I want the HEADER to change based on the value of the text box in the list.
In the text box in the header, how do I reference the text box inside the list?
To refer to any report objects you can use ReportItems Collection.
In your header textbox put a expression like this to refer to textbox in the list:
=ReportItems!TextBoxList1.Value
Remember SSRS is case sensitive. Make sure you get the right name of the text box object you are referring to.