If you are not in KDP Select, you can start uploading your books today. You have to convert your book file into a valid epub.
If you have your book in a Kindle format, you can also try a free converter like Calibre. While this software is easy to use, the files made this way not always pass the validation process.
Once you have your books uploaded, you can select which stores you would like to go to. If you would like to continue to go to Amazon directly, switch it off under your Stores and imprints section. Otherwise, you'll have to take your book down from Amazon and resend it through us. Just send us your original ASIN to keep your reviews.
Are your books in KDP Select (Kindle Unlimited)? In this case, you'll have to cancel the automatic re-enrolment and upload your books once the 90 days are over. KDP Select is an exclusive agreement and you can't go to other distributor if you are enrolled.