If you are not in KDP Select, you can start uploading your books today. You can convert your book file into an epub before uploading it, or upload it as a docx.
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 during the book upload process, under the 'Distribution' 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 or Kindle Unlimited (KU)? 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 another distributor if you are enrolled.