Click the Settings Cog.
Select Stream Administration under Posts.
Select multiple posts.
Click Find + Replace Tag (#) on the bulk actions button.
Search and select the tag you wish to replace in the Find... field.
Search and select the tag you wish to replace it with in the Replace with... field.
Click Next.
Confirm by clicking Replace Tag.
Note: The bulk Find + Replace Tag feature will not trigger email notifications for users following the updated posts or tags.