Help changing the combobox selected text to another value

Hi all,
 
This is my first post and have some experience with VB 2010.
 
Was wondering if someone could help me with a Combobox issue.
 
Basically i wish to put an Item in my Combobox such as lets say "Monday", when i push a button i wish for it to display that into a textbox as a shorter version such as "Mon".
 
So basically just looking for the combobox Item to be outputted as a shorter version.
 
Thanks in advance