How Do I Change Column Of Site In Sharepoint

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C#

SharePoint2010

i want to change sharepoint site column ...

for example i wanted to change Task Status choices with writing in eventt Receiver
but my code didn't work
i don't got error .. but task status dosen't change

 SPWeb web = properties.Web;
SPContentType contentType = web.ContentTypes["Task"];
string coulmn = "Task Status";
SPFieldChoice contentTypeField =(SPFieldChoice) contentType.Fields[coulmn];
contentTypeField.Choices[1] = "New Choices";
contentTypeField.Update();