How can I undo the last Android 7.1.1 upgrade in mi Nexus 6p?

[AC Question]

I purchased my Nexus 6p device in the USA and have been using it perfectly for a year in Argentina with a local SIM card, and after the last Android automatic update to 7.1.1, the cellular network connection has been locked because it now enables cellular network connection only in the USA and in Europe.
Google said the only way to fix this is to modify the Android 7.1.1 to its previous version.
How can I specifically carry this out ? Help!

 

[B. Diddy]

Welcome to Android Central! Follow the steps here: How to manually update your Nexus or Pixel | Android Central. Find the previous firmware here: https://developers.google.com/android/images?hl=vi