You can add or remove tags and change the year and month on several files at once, all in one step.
How to edit tags, year, and month on multiple files
Select Files in the vertical (left-hand) menu.
Select the checkbox next to each file you want to edit.
In the toolbar at the bottom of the page, select Edit tags.
In the Edit tags, month, and/or year window, make your changes:
Add tags: choose the tags to add to every selected file.
Remove tags: choose the tags to remove from every selected file.
Year and Month: choose a new value, choose Clear to remove it, or leave No change.
Select Apply.
Note: You can edit up to 100 files at a time. Edit tags isn't available for archived files.
How to replace one tag with another on multiple files
Select the files and open Edit tags as above.
Under Remove tags, choose the tag you want to take off.
Under Add tags, choose the tag you want instead.
Select Apply.