PSRenamer is a program with a graphical interface that will easily rename a group of files for you. You can change file extensions, or any part of the file name. You can also assign a series of sequentially numbered file names, or replace certain characters with others. Preview and Undo functions are available.
PSRenamer is freeware. There is no registration fee required for use of the software.
Download: PSRenamer 1.4