I have an old tablet(Lenovo Phab 2) with Android 6. I need to upgrade the Android version to 7 or heigher to run some special app for my business. I checked TWRP and Lineage OS. There were some tutorials with the links. But those links are broken now. So I came to know that this is a common scenario to all who need to upgrade their old Android device. What is the generic solution for this issue?