"Changing contect of a cell through VBA"

Question

I have created a list of 5 Categories in Column D that users can choose from. Depending on what the user selects in Column D, I have created dropdown lists in Column E that correspond to that. Now I want to code something that displays a message "Select UOM" every time the content of the Cell in Column D changes. So if a user has selected "equipment" in column D and selected "HR" in column E, and then the user changes Column D to "Expenses", the contents of the adjacent cell in column E changes from "HR" to "Select UOM". Can anyone help"