I found it, now it works,

update tabel3

set tabel3.price = dt.ac_prixVen

from openquery (mysqlapp, 'select * from product_special') as tabel3

inner join (select Ar_ref, Ac_prixVen, AC_categorie,product_id, price

from openquery (mysqlapp, 'select * from product') as tabel2 inner join

tractov15.dbo.f_artclient as tabel1

on tabel2.model = tabel1.ar_ref where tabel1.ac_categorie = '1'

) as dt on

tabel3.product_id = dt.product_id

where tabel3.customer_group_id = '1'

and tabel3.price<> dt.ac_prixVen

the mysqlapp is a linked server for mysql, and at this point my prices de prices in mysql are updated by the prices of sql

thanks