Surprisingly, Mac OS X does not contain a simple Finder command to change the file extension for a group of files in one step as a batch process. If you only have a few files to rename, doing that one file at a time is not a problem. However, when you have a large number of files to rename, the job becomes quite tedious and you yearn for an easier way. By using Automator, a free application included with Mac OS X, you can create your own automated workflow to batch rename files.
Open the Automator application. You can find Automator in your Applications folder.
Select "New" from the File menu and choose the "Workflow" template. The Automator workspace opens. The workspace displays a library of Automator actions on the left side and an area to build your workflow on the right.
Drag the action "Get Selected Finder Items" onto the workflow area at the right side of the workspace.
Drag the action "Rename Finder Items" onto the workflow area. Automator displays a warning sheet to let you know that this action will change the file names and offers to include a "Copy Finder Items" action in your workflow. You may want to add this action to your workflow for safety when you create it; you can remove it later after you test the workflow. Automator adds a "Copy Finder Items" action and an "Add Date or Time to Finder Item Names" action to your workflow.
Choose "Replace Text" in the pop-up menu at the top of the "Add Date or Time to Finder Item Names" action. The action name changes to "Replace Text in Finder Item Names."
Fill in the text replacement parameters. In the box labelled "Find," type in the current file extension. In the pop-up next to it, select "extension only." In the box labelled "Replace," type in the new file extension.
Select "Save" from the File menu and assign a name to your workflow.
Test your workflow. Select the files you want to change and then click "Run" in the toolbar. The file names change according to your specifications.
Save your workflow as an application if you want to run it by double-clicking it, rather than running it from Automator.
If including a "Copy Finder Items" action in your workflow causes it to rename the files improperly, simply delete that action from the workflow.