Wow. This sure is some serious rot here. Find out what caused it's deterioration. Water penetration from door? Carpenter ants love moisture and leave tiny tunnels in the wood. Stop this problem. It could also likely be the amount of time this piece of lumber has been in contact with concrete. Wood soaks up moisture from concrete like a sponge. That's why we have pressure treated lumber now-a-days. Anyways I won't go above ron's suggestions, because he seems like a smart guy!
All I can say it is, the rim bears the weight of the exterior wall above, which supports the weight of whatever is above it. For someone with limited carpentry experience, ya might not wanna try to tackle this.