Visit this page on Kobo and you will see a unique key. Enter it in this form, upload an EPUB file, then you'll see a download link on the Kobo.
If you tick the Kepubify checkbox, your EPUB will be converted into a Kobo EPUB using Kepubify for better reading experience.
Your ebook will be stored on the server as long as your Kobo is viewing the unique key and is connected to wifi. It will be deleted irrevocably when the key expires about 30 seconds after Kobo browser disconnects. All keys also expire automatically after 1 hour maximum.
By using this tool you agree that the ebook you upload is processed on the server and stored for a short time.