I want to change the category of a posted work from Illustration to Manga

You can change the category of an already-posted work to "manga" on the desktop version of pixiv.
It cannot be done from the mobile version or from the app.
The procedures are as follows:

＜On desktop＞

1. From your Dashboard, go to "Works" > "Illustrations" and then click the pen mark next to the title and thumbnail of the work you want to change the category for.
   
   
   
2. In the "Type of Work" category, select "Manga" and then click "Save" at the bottom of the page to change the category of the work.