It absolutely can be done. If it is load bearing, you'd need to support the supported structure while working on it.
It should be a matter of cutting them all off in a straight line, and then replacing with pieces of the same size and profile. If using wood, I would paint the replacement pieces on all 6 sides before installing to prevent future rot. Preferably though, I'd use PVC or another composite.
The hardest part is making it look seamless so you're not constantly reminded you did a patch job instead of replacing the entire boards.
Great! It’s not load bearing, so that does help...what’s best way to go about the cutting? I have various saws, but want to be sure as I basically have no idea what I’m doing. I’ll post a more complete picture tomorrow and see if it might be better to cut higher up instead...thank you!!
Any hand-saw would do it, but I would be going to my oscillating saw.
The ideal repair would be to replace the rotted portion with PVC trim board, perhaps 1.5" tall. Will last forever, and isolates the wood from the moist concrete. It would be a pretty elaborate fabrication though, because you would want the wood-grained side of the PVC board on all exposed sides. But the PVC board glues up very nicely with PVC cement. The stuff you use for pipes would work but you probably want to search for clear PVC cement (not as common).
You need to cut enough off the bottom to get above the rotted wood. Test the wood with an ice pick or knife blade. My guess is you have less than 6” of bad wood. I would make a new base box out of PT lumber (Pressure Treated) mark the height from the deck up and cut off the old and slip in the new. Caulk the joint prime and paint.
It will be easier to splice on new boards if you run some approx 1 x 4 treated boards up behind the existing boards, and glue and screw them on.
Leave enough hanging down to stop about 1.5 inches from the ground.
Then they will support the patches that you apply to match the rotted pieces that have to show.