Thank you for your quick answer!
Because I can't link my tab to the pc I use this guide:
First of all:
"Before you start. Where to download files.
Acer site - Firmwares for recovery, drivers for ADB - http://us.acer.com/ac/en/US/content/drivers"
... that's the problem, on the acer's page, opening the OS tab, I'm not able to understand which firmware I Have to download....
The tablet was bought in Italy, I don't know if it's important to know.
I feel so stupid...