There are a couple of methods you can use to edit an existing layer.
If you do not need user interaction during the edit you can simply use the EditTools property of your Layer.
The EditTools utilize a transaction system to control the addition, deletion or update of features within a Layer. For a detailed explanation of the transaction methods check out the API Documentation here: wiki.thinkgeo.com/wiki/Think....EditTools
We also have a sample application located at Desktop Edition Sample Applications - Editing Feature Layers - Add FeaturesFromAFeatureLayer that demonstrates this funtionality.
To allow your user to control the editing of Features using their mouse you would use the EditOverlay. To edit a specific feature, you first add that feature to the InternalFeatures collection of the EditOverlay.EditShapeLayer. Then to populate the features with control points (handles that the user can manipulate the feature with) simply call the CalculateAllControlPoints() method of the EditOverlay.
To see this in action check out the Desktop Edition Sample Applications - Editing Feature Layers - TrackAndEditShapes sample application.