'Подмена в строке одного символа другим.
Private Function ReplaceChar(ByVal stIN$, stCharOld$, stCharNew$) As String
    Dim l%
    
    l = InStr( 1 , stIN, stCharOld, vbTextCompare)
    While (l >  0 )
        stIN = Mid$(stIN,  1 , l -  1 ) & stCharNew & Mid$(stIN, l +  1 )
        l = InStr(l, stIN, stCharOld, vbTextCompare)
    Wend
    ReplaceChar = stIN
End Function

Private Sub Command1_Click()
    Dim stX$
    stX = "3744,55"
    Debug.Print stX
    stX = ReplaceChar(stX, ",", ".")
    Debug.Print stX
End Sub