Once you have the correct zip file, the general process involves: Unlocking the Bootloader (which wipes the device). Installing a Custom Recovery (like TWRP). Transferring the Zip to your phone's internal storage. Booting into Recovery and selecting "Install" to locate and flash the zip file. download link for a particular phone model or instructions on how to the file once you have it?
A Comprehensive Review of Android 7.1.2: Features, Benefits, and Download Process Android 7.1.2- Download Zip File
You cannot flash a system zip file on a locked device. Once you have the correct zip file, the
Android 7.1.2 is a minor update to the Android 7.1 (Nougat) operating system. It was released in April 2017 and offers several bug fixes, security patches, and performance improvements. Android 7.1.2 is compatible with a range of devices, including Nexus and Pixel devices. Booting into Recovery and selecting "Install" to locate
If you are installing a Custom ROM, you often need a separate "GApps zip file" to get the Play Store and Google services. OTA (Over-The-Air) Zips:
S
Source: Wikimedia Commons - http://upload.wikimedia.org/wikipedia/commons/3/36/2008_Europe_Political_Map_EN.jpg
Source: Wikimedia Commons - http://upload.wikimedia.org/wikipedia/commons/9/9c/European_union_future_enlargements_map_en.png