Sometimes you might need to edit multiple posts, perhaps to tag them with a new category or a keyword, or you might need to change authors, turn on or off comments, etc.
Rather than editing each post one-by-one, you can go to your Dashboard and click on All Pages. Select the posts you want to edit by clicking on the square selection box to the left of the title, then click on the Bulk Actions/Edit drop down menu item, and click on Apply.
You should then have the option to add or remove tags or categories, as well as change the author, change commenting options, or change the status of the posts, amongst other things. Hit the Update button after you’ve finished making those changes.
The same thing can be done with your pages, though with a few less options.