rotted sill plate

[what'sahammer?]

I have about 8 feet of rotted sill plate. Do I need to replace the entire length (about 25 feet) or can I just replace the rotted portion?

Also there is rot in the rim joist. How do I replace that?

I really don't want to use a hydraulic jack but I'm willing to do anything to fix this damage.

BTW, we are having an exterminator come out tomorrow to check for termites just in case it is not rot. Either way, the portions of the rim joist and the sill plate will need to be replaced.

[pushkins]

If you only need to replace about 8' of rotted sill plate then it's not so bad.
You will need to support the floor joists while you remove and replace the sill plate , how much you replace is up to you but you need to somehow remove the section you choose to replace, most often it is easier to replace the length of lumber that is already there. (with ACQ lumber).
You might be able to cut (with clean margins) the rotted wood out and then just replace the cut out section. Make sure that you define "rotted" wood and then cut back 4-6" from that into solid wood.
If your only replacing 8 or so feet then 2 simple jacks with a 4x4" across the top under the floor joists would suffice while you cut out and replace the lumber.

[what'sahammer?]

Thanks for the advice Pushkins!

I there a good way to replace about 8 feet of the rim joist? Unfortunately, the floor joist are nailed to this portion of the rim joist.

[pushkins]

If you have full access to inside and outside of the rim joists it's pretty easy.
Determine the rotted area to be removed, set your circular saw to the rim joist depth (often 1 1/2 ") and cut down a line in the center of a floor joist furtherest out from the rotted section. Pry off the rotted rim joist and replace with a new treated one. You may have to cut off any nails/screws that may be coming down into the rim joist from the sub floor, if that's the case a reciprocating saw will do the job cutting the nails/screws.