If replacing the front retaining ring only
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Use a screwdriver to remove the old retaining ring.
Clean the mating surface.
Install a new retaining ring, using die 9995455 ; grease the retaining ring.
Push-in the retaining ring using the center nut on the crankshaft.