As disclosed in the preprint/open peer-review settings when authors submit their paper, preprints cannot be "unpublished".
Preprints receive a DOI, are submitted to various databases such as Crossref, and are considered citable. Because preprints do not "count" as formal publication, there is no "withdrawal" process. Preprints (including submitted manuscripts) will remain on JMIR Preprints in perpetuity and will link to the version of record (VoR) when and if the paper is published formally in a peer-reviewed journal that uses Crossref.
In exceptional cases (e.g. clearly wrong information that constitute a public health concern) we can add a new cover page to a preprint PDF that contains an explanation from the authors explaining why the preprint was withdrawn. The originally submitted preprint will still be part of the PDF, but the withdrawal explanation will be on the first page. Please submit an explanation and a signed PDF to firstname.lastname@example.org.
We will not accept withdrawals that are motivated by strategic considerations such as "I want to submit it to another journal and they may turn it down if they see that it was already published as preprint.". Note that this concern is factually wrong: The vast majority of journals and publishers encourage preprints and will not reject submissions for that reason, see https://en.wikipedia.org/wiki/List_of_academic_journals_by_preprint_policy.