You should remove the items from your source collection and the grid will pick this changes and update its UI.
For example if you want to remove the selected item you can use the following code:
If you want to add an item as a first row you should insert it in the collection that the grid is bound to:
assuming that your object is MailMessage and the source collection is ObservableCollection.
Also you may want to look at our Step-By-Step tutorial for Silverlight
illustrating this scenario as well as many others.
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