Sub Test()
Dim assemDoc As AssemblyDocument
Set assemDoc = ThisApplication.ActiveDocument

Dim assemCompDef As AssemblyComponentDefinition
Set assemCompDef = assemDoc.ComponentDefinition

Dim occ As ComponentOccurrence
Set occ = assemCompDef.Occurrences(1)

Dim partDoc As PartDocument
Set partDoc = occ.Definition.Document

Dim partParams As Parameters
Set partParams = partDoc.ComponentDefinition.Parameters

Dim paramName As String
paramName = "LarghezzaVasca_Ex"

Dim paramX As Parameter
Set paramX = partParams(paramName)

For i = 1 To 5
Dim newExpression As String
newExpression = InputBox("Enter new value for " & paramName, paramName, paramX.Expression)

If Len(newExpression) > 0 Then
    paramX.Expression = newExpression
    Call assemDoc.Update
Else
    Exit For
End If
Next
End Sub