I want to edit the settings for an already-posted novel series – pixiv Help Center

You can alter the settings for a novel series from the edit novel series page.

Please go to your Dashboard and then Works > Novels > Series.
Click the pencil mark next to the series you'd like to edit.
You can make changes from the edit novel series details page.
Finally, click "Save changes" on the edit novel page page to update the novel content with your changes.

Please go to your "Manage works" page and open the "Novels" tab. Then tap on the "Series" displayed on the cover of the work you'd like to edit.
Otherwise, you can click the "Novels" tab on your profile page, and then tap "Novel series" by the work you'd like to add to.
Tap the pencil mark for that novel series' page.
Change the settings from within the editing page.
Finally, tap "Save changes" on the edit novel series page to update the novel content with your changes.

Currently, series settings cannot be edited via the pixiv app.