Update selected ListItem from an Dropdown in the SPFieldChoice

How can i select a value without replace or change the values in the SPFieldChoice.

I'm using this code to update the ListItem but all it make is replace the first option on the field with the value i am getting in the dropbox selection.

ListItem["MyField"] = dropdown.SelectedItem.Value; 

Let me be more clear...

FieldOptions:

A

B

C

i want to set the option C to be the first show to user whem they see the list.

all i get after use the code is 

FieldOptions:

C

B

C

Greetings from Brazil.