You can delete your reading progress by deleting your browsing history.
In the browsing history page, novels tab, if you click "Clear history", your entire browsing history will be deleted, along with all the reading progress saved up to that point.
* "Clear history" is accessible to everyone, regardless of whether or not they are Premium members.
* This feature is not yet available on the pixiv app.
<Desktop version - Browsing history>
<Mobile version - Browsing history>