Should I replace the area in the rough window sill that has the rot?

[mguise]

Hello community. I am replacing a couple of windows in my garage. I have zero experience but I’m slightly handy and crazy enough to do this myself. I have a question though. A while back I removed my old rotten window then I put a board over the window and procrastinated on starting this project. In the meantime rain came in and caused some rot to the rough sill. I uploaded some pictures for you to a look at and get your opinion. Do you think I need to replace the area that has the rot in the rough window sill? Or, because it’s minor rot, and it’s the rough sill, can I let it go and start installing my window? If I need to fix it, stupid question, I’m not sure how to go about this because the rot is in a 2x4 that runs from the floor to the ceiling. How would I fix that without having to replace the entire 2x4. Most of that board is behind drywall. Thanks in advance for your help.

 

[Irv]

I wouldn't call that minor rot. Replace it.