Bonjour, après 2 jours de recherche internet, c'est microsoft qui m'a donné la solution :

Sub UpdRChevauxCotes(Numero As Integer, cote11h As Single, coteDef As Single)

Dim MyCle1 As Variant
Dim MyCle2 As Variant
Dim MyCle3 As Variant
Dim MyBool As Boolean
Dim mystr As String

    
    On Error GoTo adoerror:
    
    MyCle1 = Reunion
    MyCle2 = NumCourse
    MyCle3 = Numero
    
    Set RChevaux = New ADODB.Recordset
    
    With RChevaux
        'Select the index used in the recordset.
        .Index = "PrimaryKey"

        'Set the location of the cursor service.
        .CursorLocation = adUseServer

        'Open the recordset.
        .Open "Chevaux", cn, adOpenKeyset, adLockOptimistic, adCmdTableDirect

        'Find the customer order where OrderID = 10255 and ProductID = 16.
        .Seek Array(MyCle1, MyCle2, MyCle3), adSeekFirstEQ

        'If a match is found, print the quantity of the customer order.
        If Not RChevaux.EOF Then
            RChevaux!cote11h = CSng(cote11h)
            RChevaux!coteDef = CSng(coteDef)
            RChevaux.Update
        End If
    End With
    
    RChevaux.Close
    Set RChevaux = Nothing
    
    Exit Sub
    
adoerror:

    MsgBox ("Erreur de mise à jour de la cote du cheval " & Numero & " de la course " & NumCourse)
    Call EcritSurEcran("Erreur de mise à jour de la cote du cheval " & Numero & " de la course " & NumCourse & vbCrLf)
    
    RChevaux.Close
    Set RChevaux = Nothing
    
    FlagErr = True
    
End Sub