First, select the files whose extension you want to change. You can either use the file-selection dialog or simply drag the files onto the user interface.
In order to add this action to the list, simply click on the button featuring the Plus (+) symbol and select from the appearing popup menu 'Other -> Edit file extension'.
Select now the entry Change from the actions menu and enter the new file extension. In the example shown, the file extension 'JPEG' is to be replaced with 'jpg'. The dot in front of the file extension can be omitted.
The new file extensions are displayed in real time in the preview list and thus make it possible to check the result in advance.
Now, the Tasks dialog can be opened by clicking on the Apply button. The files to be renamed are listed there, and the renaming process can be started by clicking on the Start button.