Rename inside folders

Hi. I have 1400 folders. Each folder contains 4 files. The file is, for example, 19060503 but it should be 1906-05-03. I have been able to rename the folders, which had the same error. But I don't know how to tell the program to make this change in all the files in all the folders without having to do it one by one.
Any suggestion?
Thank you

Reply to #1:
Select the "Rename Files" tab, then "Add" -> "Directories" or open the Folder Panel.