You need to access the hard drives with a computer. As long as the Aurender is connected to your network it’s pretty easy.
On your your Aurender app, you’ll be able to find the name of the device. I called mine “AurenderN10”. If you have a Mac, go to finder and hit command-k. That’ll open a window that allows you to reach a network drive. Type in smb://NAME. Name=the name you created for the Aurender.
That gives you access to the hard drives. You can then delete files. However I’d recommend never adding files this way. I encountered issues with that, and found the app works best to copy.