How can I modify the title of work items in TFS en masse?

It is possible because when selecting a WorkItem, in the half screen on the bottom, you will have its details, and thus can change it. After you re done, you will be able to save all in one Ctrl-S

Better way :

I have done that by exporting result of query in MS Excel, modifying multiple columns and rows rapidly; then Publish the whole by one click.

For Visual Studio 2010, 2013, the following will work:

1. Open a list of the items you wish to edit in Visual Studio/Team Explorer Tab. This can be done via a new query.
2. Highlight all items (of the same type) that you want to change.
3. Click the ‘Open in Microsoft Office’ drop-down or right click on one of the selected task items and select ‘Open In Microsoft Excel’.
4. Edit the fields you are interested in using standard Excel functionality to copy/paste or replace.
5. Click the ‘Publish’ button at the upper left in Excel.
6. After that has completed, click on the Refresh icon at the top of your product backlog view in Visual Studio.
7. Your updated/bulk-edited entries will now appear.

You can bulk edit in Team Web Access 2010. Opening the query in Excel works better for bulk-editing the titles, but if you're going to change, say, Area Path, TWA provides expandable/collapseable nodes in the hierarchy, while Excel simply gives you a drop-down menu of values.