Presentation

In the directory you've chosen, select the files by clicking their checkboxes
(the "Sel" column contains the selection number following the order you've done it)

Siren1

In the "Expression" combo, enter a renaming expression, validate it with the "Enter" key.
In this case, "%n" corresponds to the selection number and "%e" to the file name extension.
The "Future name" is immediately computed.

Siren2

Click the button with the "Play" shape, the renaming is done.
The "Undo" button has become enabled.

Siren3