This is my experience to update Mini CN900, I hope it helps you.
Free Download Smart CN900 Mini 188.8.131.52 update tool with Win7 Win8 driver:
Connect mini900 with laptop via USB cable
Open driver folder, install driver (for Win7 or Win8 system)
Next,find Computer on the desktop,Set com port to COM4
Run CN900 Mini firmware update tool on desktop
And click update
Next, click CN900 MINI, click update
The firmware has updated to the new version.