Go to your thread. In the upper right corner, above your first post, see the label marked "Thread Tools". From the drop-down menu, choose "Edit Thread". The title field will open, and can then be changed.

The same steps are done to change the prefix to your thread, such as marking a thread Finished in the marketplace, or changing a description prefix in the Galleries/Photostories or other areas.

You cannot change the title of other people's threads. If you feel a change is needed, contact the moderators in Ask the Moderators.