LineageOS is the gold standard for custom Android 6 ROMs.
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To download or install Android 6.0 (Marshmallow), you can find and ISO files through various official and community-archived sources. 1. Official OTA Update ZIP Files LineageOS is the gold standard for custom Android 6 ROMs
Ensure your device has at least 50% charge before starting the process. android 6 download zip file top
Example with ZArchiver: