You can only change the category of a manga work on the desktop version of pixiv.
It cannot be done from the mobile version or from the app.
The procedures are as follows:
- Please go to your "Dashboard" > "Works" > "Manga" and then click the pen mark to the right of the title and thumbnail of the work you want to change the category for.
- Select "Illustrations" in the "Type of Work" section and then click "Save" at the bottom of the page to change the work's category.