How do I change DetailsView data with a DropDownList?

[Dossman]

I am somewhat new to ASP.NET. Classic ASP/SQL Server experience mostly.
I have a DetailsView control with a DropDownList on an
EditItemTemplate. The DropdownList shows in place of a bound Textbox.
The idea is to show the user the data in a bound textbox field, but
show a dropdownlist on Edit. The Dropdownlists' datasource is the
vw_aspnet_membershipUsers table in the database showing only the
username field. The selected data needs to go into the textbox field
which is bound to a different table altogether. Here is wherer my
problem lies.
#1. How do I default the dropdownlist value to what's in the textbox?

#2. How do I get the selected value into the correct field for
updating?

Thank you in advance.

John