In order to Batch Update Tags:
Navigate to the Inventory > Products Tab
Select the Category you would like to Batch Update from the Category Tree to the left
Select The Batch Update > Products Tab in the lower right corner
Check the box next to the grayed out "Add"
Type in your tag and select "Batch Update"
Note: Batch updates can take anywhere from 5 minutes to a few hours depending on the number of products being updated and server traffic.