Are you trying to “rename” an existing tag? If yes, under CRM, Settings, then Tags you may search to find the existing tag. Then click the name of the tag to view it’s details. In this view you may rename the tag simply by editing the Tag Name and hitting the Save button. This changes the name of the tag everywhere it exists.
Or, are you trying to add a new tag to an existing contact? If yes, at a minimum you need two columns in your CSV file: Isoft ID and the Tag Name. When you map the columns during the Data Cleanup process (vs Import Data) you’ll see an option to create new tag… at the top of the available fields to map to.
If you’re trying to import 400 new unique tags without actually applying them to any contact records you could import a dummy contact record with all the tags. First, Transpose your csv doc from 400 rows to 400 columns of data (copy, paste special, transpose). Then when you “import” you’d use the create tags from the data option when mapping each column. You’d need to map all 400 columns but would be easier/quicker than manually creating each tag.