Thanks! If any other users could confirm that this is the correct method, that would be great!
In order to install the firmware you need RK Batch Tool in your PC included in the rar package. Rockchip drivers are also included in case you don't have them.
Open Batch tool and select the Legacy_XX image file (.img).
With GPD power off, just connect it via USB Cable to PC. Then just press and Hold Volume + button and Press Power. RK Batch Tool will detect device, and you'll see the number 1 with a green box. If you see with a pink box, just select Switch Button until you see it in green. If the system ask you for some drivers, then just use the one included in the Batch Tool (or using driverInstaller package to install rockchip USB drivers). It's not mandatory but recommended to install with full battery.
Once device is detected with a green BOX, just select Restore and wait patiently while firmware is written.
Device will auto reboot to recovery and format your partitions. Just wait patiently until all process is done, since it's a long process and first boot can take several minutes, so just wait until it finishes and you see the home Android launcher.