Overwriting an auto populated text field from a drop down box.

I have a form that I am building that has a drop box that will auto populate a text field based on what the end user selects..  I also need the user to be able to overwrite that information with their own in the populated text field.  I’ve searched, but cannot find a solution that works for what I need.  When the populated text is overwritten, it defaults back to  the original text.  Any help would be appreciated., thank you.