Update single record with value of dropdown list

See more:

ASP.NET

database

C#7.0

hi all
i have a drop down list connected with sqldatasource
and fill with some "Room Code"
i want
when user select one of these code's and click on Ok
this should happen:
find the record that has St_Code = session["stcode"] //its get us who want to get the room?
then when it founded set or update column Room_Code of that record to dropdown selected item
its really simple way to say it :D
what should i do ? if you need to see my codes i will send it but im not sure need it!
i just want to update a single record with the value of dropdown list
for more understanding
ST_CODE = Student Code that means student registered number
and ROOM_CODE = Student room code that means wich room for wich student registred

What I have tried:

nothing know and i've not trying any thing yet
and here is my dropdown code i just connected to the database

SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["DatabaseConnectionString1"].ToString());
protected void Page_Load(object sender, EventArgs e)
{

}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{

}