Automator is pretty straight forward...
(1) Fire up Automator
(2) Select all the files you want to rename and drag them into the workflow...
(3) Ensure "Actions" is selected and type "rename" into the search box...
(4) Drag the "Rename finder items" action into the workflow area below the list of files. You'll get a dialogue asking you if you want to create new files (copies) or alter the originals. You can choose either, in which case you'll need to select a destination folder. For this instruction, I'll choose "Don't Add", i.e. Don't add a copy function into the workflow...
(5) In the action block, you'll need to change the "Add Date or Time" drop down to "Replace Text"...
(6) Enter details of the wrong file extension to be found in the file names, and the replacement extension you want...
(7) Click the "Run" button...
(8)Hopefully you have a nice set of green ticks and the file names have been changed as you wanted. You can check what the Rename action has done by clicking the "Results" button...
Good luck!
