DSO nano V3 return to original firmware

I’m using a DSO nano v3 (Serial:8E31A799, Licence:F98F9C4F).
And I tried to change a firmware from original to Benf.
But it doesn’t work and my DSO have stuck on firmware screen.
Therefore, I’d like to return original firmware.
How should I do?
I would appreciate it if you could tell me the method.