You will need to get the correct object type.
{code}
Sub ModifyLine(ByRef tr As Transaction, ByVal objID As ObjectId)
Dim e As Entity = tr.GetObject(objID, OpenMode.ForRead)
If TypeOf e Is Line Then
Dim l As Line = tr.GetObject(objID, OpenMode.ForWrite)
Dim sPt As New Point3d(0, 0, 0)
Dim ePt As New Point3d(1, 1, 0)
l.StartPoint = sPt
l.EndPoint = ePt
End If
End Sub
{code}
Then commit your transaction at the end to save the changes.
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(defun botsbuildbots() (botsbuildbots))