How to edit existing expression?

Imports System
Imports NXOpen
 
Module create expression
 
    Public theSession As Session = Session.GetSession()Public workPart As Part = theSession.Parts.Work 
    Sub Main() 
        Dim leftPart AsString="12"Dim expression1 As NXOpen.Expression=NothingDim thr_exp As NXOpen.Expression=NothingDim expression_a AsString="thr_txt" 
        'create expression 
        TryDim theExp As Expression = workPart.Expressions.FindObject(expression_a) 
            workPart.Expressions.Edit(theExp, "M""+stringValue( "& leftPart &" )") 
        Catch ex As NXException
            ex.AssertErrorCode(3520016) 
            expression1 = workPart.Expressions.CreateExpression("String", expression_a &"=""M""+stringValue( "& leftPart &" )")EndTry 
    EndSub 
    PrivateSub Echo(ByVal output AsString)
        theSession.ListingWindow.Open()
        theSession.ListingWindow.WriteLine(output)
        theSession.LogFile.WriteLine(output)EndSub 
    PublicFunction GetUnloadOption(ByVal arg AsString)AsIntegerReturn System.Convert.ToInt32(Session.LibraryUnloadOption.Immediately)EndFunction 
EndModule