By hand
In the admin
- Open Posts or Pages, and narrow the list with Status, Language, Author, Tag, Access or a search.
- Tick the posts you want. Once one is ticked, Select all takes every post the filters find.
- In the bar that opens, choose Authors to replace or add authors, or Tag to add or remove a tag.
- Choose Access for Public, Members only or Paid members only, or More to feature, stop featuring or delete.
- Press the change in its menu: Apply, Add tag, Remove tag or the verb under More. The list says how many posts changed.