Pass Value From Unbound Textbox To Bound Textbox

Hi: There are two textboxs in my main form. One is bound and another is unbound. There is no entry in the unbound textbox as values come into automatically after entering some information in the subform. My question is how to i pass values from unbound textbox to bound textbox every time when the value change in unbound textbox i need to change the value in the bound textbox. When the form load there is already value in the bound textbox which i want to override based on the values from the unbound textbox.

Thank You.