Assign Taxonomy Terms to Selected Nodes Using Views (Mass Update Fields)

If you want to assign taxonomy terms to a list of nodes at once in you can do it using Views and Views Bulk Operations.

Assuming that you know views and you have already created a view of nodes which you wish to "tag" with specific node terms. If necessary add the filters to the view to narrow down the exact nodes. Make sure to add permissions to the view so that only administrators can access it in the case of sensitive data.