So the first PC games to support AMD FSR 3.1 are Ghost of Tsushima, Marvel's Spider-Man Remastered, Marvel's Spider-Man: Miles Morales, Horizon: Forbidden West and Ratchet & Clank: Rift Apart.
As Nixxes noted, AMD FSR 3.1 offers improved FSR upscaling with less flicker and better detail preservation compared to previous versions. In addition, AMD FSR 3.1 allows everyone to use AMD FSR Frame Generation along with other upscaling techniques. Thus you can use it with NVIDIA DLSS and Intel XeSS.
AMD FSR 3.1 also includes a Native AA option that uses high-quality anti-aliasing without upscaling for improved image quality. Think of it as AMD's answer to NVIDIA's DLAA. To enable this option, set the upscaling method to Off and select AMD FSR 3 Native AA from the anti-aliasing options.
As always, Steam will download these updates the next time you launch their clients.