Amazon has finally updated functionality to let us reset the furthest page read on our Kindle books. For a person like me who re-reads books all the time, this is long overdue, but greatly appreciated.
The Learn More link takes you to this Kindle Help page advising the below:
Reset furthest page read
The furthest page read feature keeps track of how far you’ve read in your Kindle title across all devices registered to the same account. This allows you to seamlessly switch reading from one device to another and pick up where you left off in your content. You can reset your furthest page (or location) read on your Kindle title using the “Reset furthest page read” option in Manage Your Kindle. This can be helpful if you’ve skipped ahead in your content or are re-reading it and now are having trouble keeping all of your devices in sync to an earlier point.
To reset your furthest page read in Manage Your Kindle, click the Actions button next to the title and choose “Reset furthest page read” from the dropdown menu. The next time you open that title on a device, the page where it opens will become the new furthest page read that syncs across all your devices and apps.