Option Explicit
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim S1 As String
    If Target.Column =  1  Then
        If InStr(Target, "=") <>  0  Then
            S1 = InputBox("введите новое значение")
            If S1 = "" Then Exit Sub Else
                Target = Left(Target, InStr(Target, "="))
                Target = Target & S1
        End If
    End If
End Sub

Option Explicit
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
    Dim S1, S2 As String
    
    If Target.Column =  1  Then
        If InStr(Target, "х") <>  0  Then
            S1 = InputBox("введите значение 1", "модель Х")
            S2 = InputBox("введите значение 2", "модель Х")
                If (Len(S1) =  0 ) Or (Len(S2) =  0 ) Then
                    Exit Sub
                Else
                    Target = Left(Target, InStr(Target, "х"))
                    Target = Target & S1 & "x" & S2
                End If
        ElseIf InStr(Target, "=") <>  0  Then
            S1 = InputBox("введите новое значение", "модель =")
                If (Len(S1) =  0 ) Then
                    Exit Sub
                Else
                    Target = Left(Target, InStr(Target, "="))
                    Target = Target & S1
                End If
        End If
    End If
End Sub