I'll do my best to explain, here. If you have any follow-up questions, I recommend hopping on an Advanced Q&A webinar: http://webinars.clarizen.com/
To achieve this, you can write a new custom action at the Project level. Essentially, in the "Set Actions" portion of the custom action, you'll want to choose the "Update Field" option, choose the "Work Items > Name" as the field to update, click the checkbox to "Filter Related Items" and put your filtering criteria in there. This, again, is based on your requirements and the input you obtain from the user. Perhaps you can update all tasks that contain a specific sequence of characters in the name, i.e:
where "XYZ" can be hard-coded, or asked of from the user.
Again, hop on an advanced webinar for further clarification.