There are a few possibilities; one would depend on what kind of a wall finish you have on exterior and how easy (if possible at all) it could be removed to access this deteriorated bend joist. This way you could replace the entire damaged section at once. This could be the easiest with vertical types of siding boards and other wall finishes that can be disassembled without damaging them.
If exterior access is out of the question, you’re left with cutting out rotted sections from between the joists and replacing it with new boards (I’d recommend pressure treated wood). You can remove existing board by chiseling it along the floor joists in each joist space, using a Sawzal, Rotozip, or any other tool you may have available.
In case there’s a double board or some extra space behind the band joist (between the joist and the exterior wall finish), you might be able to cut one longer (full required width that fits behind 2-3 joists) replacement board from a thinner, treated plywood, and try to slide that longer piece behind the floor joists (one piece at a time / 2 or 3 layers of that thinner plywood to match original thickness).
Depending on the design of your floor frame this may or may not work. If one, full width piece will not fit, use smaller sections to fit between the joists.
Another option is to remove any loose or heavily deteriorated sections of boards, allow the remaining wood to dry completely, and install new boards over the old ones just to cover the holes.
All of the above can be applied only if this bend board isn’t supporting your floor joists; the floor joists are fully supported by the foundation or other structural formation. If it acts as a ledger board / floor joists have been improperly installed (too short – look at a drawing below) you need to first support them properly so they will not depend on the band board’s integrity.
Let me know if you have additional questions, just click “comment” below