WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Belt Tensioner Mounting Screws
Courtesy of VOLVO CARS OF NORTH AMERICA.
Remove:
both mounting screws from the auxiliaries belt bracket for the belt tensioner
the belt tensioner. Install the new belt tensioner. Tighten.