I created an Editor Utility Blueprint for that.
You can get it here, for free: https://artstn.co/m/M066
UE4 Editor Utility Blueprint (aka Blutility) for renaming multiple files at once.
Supported Unreal Engine Versions: 4.24 - 4.25
Usage demo: UE4 Editor Utility Blueprint (aka Blutility) for renaming multiple files at once, ver. 3.2 - YouTube