Change Post Author

The author you want to assign the post to needs to exist as a user. so make sure they have been added first.

Find the author dropdown and select the new author

Click 'Update'

Make sure the Author metabox is enabled in Screen Options

Navigate to Author Metabox and select new author from drop down

click 'Update'

Navigate to the Post or page in the Dashboard index

Hover over the post to activate the quick links

Select "Quick Edit"

Switch the author from the drop down list