You can rename tags from the desktop version of pixiv only, and not from mobile or app.
- From the home page, click on your icon in the top right corner, and then Bookmarks.
- Call up the bookmark tags list by clicking on the ・・・ icon on the right-hand side, or below the bookmark tags.
- Click on "Edit tags", and you will be prompted to the editing mode.
- Click on the pencil icon for any tag to rename it.