This actually isn't as complicated as it first seems.
First, you're gonna have to toss the mobi. It's proprietary and cracking it open is an exercise in futility.
Start Sigil.
Open the epub in Sigil.
You should have the book browser window open and the WYSIWYG window open (I think the TOC is also open by default, but not sure).
In the book browser window, look under text. Depending on how the formatter did it, you should either have one giant file or several small files/chapters.
If you already have a back matter page, use that page and just update a line.
Or you can add a brand new page.
Go to File > Add > Blank HTML Sheet
Don't worry you can change the name.
Now you probably also have a style sheet (hopefully it's separate). Create some basic HTML (there are online HTML Editors available to use for free), copy and paste it into that new file, use the existing styles in the style sheet and voila, you have an updated ebook. Change the name if you want. Hit save.
You probably want to add it to the TOC, but that's not too difficult. You can add it in through the WYSIWYG window, then flip to the code side and add a link. Again, hit save or save as, and you have an epub you can upload to KDP without a problem. (Amazon will convert the epub into a mobi and if the only thing you are changing is a page in the backmatter, it shouldn't be a problem.
Sigil only looks complicated. But once you step back and see what it does for you (it will update the content file automatically) and how you can make changes on the fly, it fast becomes a tool you wonder how you ever lived without. Seriously, I embed fonts with Sigil, make fast edits in Sigil, update backmatter pages in Sigil, pretty much all of the upkeep gets done in Sigil. Plus, Sigil is a great place to test out a CSS file or to find stupid mistakes that you can't quite figure out why/how they happen.