ComboBox and textbox

Question

I designed a change password form and would like to give this option to Admin to see full name of user on comboBox and when he selected the user, username appear on the Label or Textbox, 

Table User:

Firstname:

Lastname:

Username:

Password:

Email:

Notification

Combobox--> show Firstname + lastname

Lable : Display username belong to selected item value from combo box.

Please share your idea.

Thnak you