status is not changing

hi all,

I have one drop down field Status,

                  which contained different values like

                   new, send to customer, create upadte/change, completed....

now, i am choosing the status = 'update/change' and clicking on 'save' button

after save my status = 'update/change' only.

but as for my requirement after save is should be like

                           status= completed.

plz give me some usefull suggestions,

thanks

--santhosh