This article explains how to edit covers for already-posted novels on pixiv mobile. On desktop, the appearance may differ slightly but the features available are exactly the same. The pixiv app does not currently support this feature.
If you used a cover template
If you picked a cover from the templates, you can click "Edit cover" from the "Cover preview" tab on the edit page to edit the title and other parameters.
*Some templates do not support the "Edit cover" feature.
If you can't click/tap on the "Edit cover" button
For cover templates that you cannot edit using the "Edit cover" button, re-select the template to edit it.
If you uploaded your own image for the cover of your novel, you cannot use the "Edit cover" feature. Instead, please use the "+" button to re-upload the image you would like to use.