to change text in textbox

Question

hellow, i can't find how I need to change the text in my textbox when: I click on an item in my listbox then that word is in my textbox, but I can't change the text. I just can change it when i click on a button "Change". Then my textbox must be unlockt sow I can save things. dos somebody know how to do that? thx and greetings