How to replace rotten OSB subfloor under Patio Door

Hi all. I'm new to this site and am hopeful someone can give me an easy way to go about dealing with this. While prepping for A hardwood flooring install I found an old water leak and so I've removed some of the flooring but am now stumped. It's winter here so wondering if there's an easy way to replace the piece of OSB running underneath the door sill? I tried to attach a picture but I kept getting an error. Anyways, I'm open to any ideas! :confused: