Before attempting any manual/optional updates for your Retroid Pocket 2, please download and install the drivers below:
These drivers will allow your PC to talk to your RP2 while the console is on and running the Android OS. Download the zip file, extract it then right click on andrdoid_winusb.inf and click "Install".
These drivers will allow you to use SPFlash to load/update your firmware.
For all current firmware upgrade files and guides, including stock app restoration packages and root solutions for 6.0, 8.1, and LineageOS, go to this link. You will find the guide (and required software) in the START HERE: Prerequisites and Guide folder.
And remember: please follow the guides closely or it may affect your mood!
For instructions on rooting your device, see Root