When Samsung Galaxy S3 launched it comes in two colors – marble white and pebble blue, my favorite one was marbled white. The color is not the only thing that made this smartphone the best smartphone of that year, latest and the most powerful hardware (of that time) made this phone the coolest and the most powerful device of that time.

Also, that was the first when Samsung used this smartphone design which is now the identity of Samsung smartphones. Now this is all old news, for you and also for Samsung that’s why it is not supporting it anymore for Android version update but don’t you loose hope we still have some developers who always works for devices like these. Today I have a right custom ROM for you to install it on your phone and update your AT&T Samsung Galaxy S3 phone to Android 6.0 Marshmallow.

Here is everything you need to install an unofficial version of Android 6.0 Marshmallow on the Samsung Galaxy S3 with the model number from AT&T by installing a CyanogenMod custom ROM:

Things to Remember:

  • Root your phone to get the system files access and install a custom recovery on it (TWRP recovery recommend).
  • You need to charge your phone to at least 70% battery so it won’t go off during the installation process.
  • Backup your critical data before installing any custom ROM because it will wipe all the stored data.
  • This installation process is for only AT&T Samsung Galaxy S3.

Steps to install CyanogenMod 13 on AT&T Samsung Galaxy S3:

  1. Download CyanogenMod 13 custom ROM zip file from here.
  2. Download Google apps zip file from here.
  3. Connect your AT&T S3 phone to your PC and transfer both the zip files to your phone.
  4. Disconnect your AT&T S3 phone from PC and turn it off completely.
  5. Boot up your phone to recovery mode (boot it by pressing and holding volume up key, home key and power key together).
  6. When in recovery mode tap on the wipe and confirm to erase the internal memory.
  7. Now come to the main menu and tap on install, select the CM 13 zip file and confirm to flash the custom ROM on your phone.
  8. After that tap on install again and flash the Google apps zip file this time.
  9. After installation tap on “reboot” followed be “system” to restart the phone to OS mode.
  10. The phone will take about 10 minutes to boot for the first time but after that, it will boot normally (faster than before). Use comment section for all the queries and suggestions.

In conclusion, that is how to install Android 6.0 Marshmallow unofficially on the AT&T Samsung Galaxy S3 smartphone by swapping the stock ROM for a custom ROM. Since the stock, ROM stopped rolling out much before the custom ROM, the only way you can ever get the Marshmallow updates on your S3 handset is by installing one of the custom ROMs like we have here that is made by third-party developers. A third-party developer is a person or group of individuals who are not associated with Android at all. The way in which we make that happen this time at least is by installing CyanogenMod 13–one of the latest builds of CyanogenMod available at the date of writing this guide.