The sounds loaded on the RØDECaster Pro's sound pads can be replaced in the pad settings menu on the RØDECaster Pro (press the musical note icon on the home screen) or within the Companion App.
You can either record over the audio directly via the RØDECaster Pro or load a new file from your computer via the App.
To learn more, check out the RØDECaster Pro User Guide page on Using Sound Pads.