Save info from the form to a text file

I am saving information from a textbox to a text file using vb.net windows application. but everytime i enter a new word in the textbox and hit save, then close the program, it will ask if i want to update the details if i say yes then it clears the old information stored in the text file and saves the new info.

how do i make it update the text file without clearing the old ones. so i want the first information entered to be in the text file and the new info entered to show under the old ones.