How can I display default value of dropdown in "Box 2" if "Box 1" is blank

[troyt41236149]

Box 1 and Box 2 are dropdown boxes with the first value as default which is a blank line. I would like the Box 2 value to show the 2nd item in the dropdown list ("E") by default anytime Box 1 has been changed to another value than the default or blank line value.