XSARA PICASSO milage correction

Got here a 2006 Xsara picasso, they fitted used I.C from scrap yard, original vehicle milage is 152.030klms, the new I.C is displaying 350.000, I know the BSI will now save the higher milage and will also need sorting.
They would like it putting back to its original milage, whats the procedure for sorting this

Thanks