Hi & welcome. It sounds like you are talking about the rim joist. This repair is done from the outside. It is not a hard repair, but IS time consuming. You need to open up the lower part of the siding to get access and cut out the bad sections, sister in new sections.
You can do a search here and find other posts on the subject.
"If I can't fix it, I can screw it up so bad no one else can either."