Hello IsabellaCifoni, how are you?
My name is Lílian, I am an independent consultant and I am very happy to help you today.
In this case I recommend that you contact the Tablet Manufacturer. Microsoft has not yet authorized the installation of Windows 8.1 Pro and Windows Rt 8.1 on portable devices.
The only portable device that has Windows officially installed would be the "Surface"
Microsoft Tablets:
https://www.microsoft.com/en-us/surface
I do not recommend formatting this device without first consulting the manufacturer, as it may be an OEM copy. And OEM copies can only be installed or supplied by the Manufacturer.