The OnePlus One is famous its Cyanogenmod 12S operation system but a big problem is the speed in which new updates are rolled out to the OnePlus One. Currently, the latest Android 5.1.1 version isn’t available on the OnePlus One but the OnePlus One can be updated with an unofficial Android 5.1.1 update OnePlus One using SlimLP ROM.
Bellow is an in-depth guide on how to install Android 5.1.1 On the OnePlus One using the SlimLP ROM (SlimSaber-bacon-5.1.1-20150604.zip) the latest version includes several additional changes, here is a full list of the SlimLP changelog:
Before upgrading your OnePlus One to Android 5.1.1, Install Custom ROM or modify Android in any way we always highly recommend you create a full system backup. There are several methods to Create a Backup in Android 5.0 Lollipop, bellow we will go step by step on how to successfully create a backup and restore it in case something goes wrong while updating OnePlus One to Android 5.1.1:
Create Android 5.1.1 Lollipop Backup: We have already covered this in an in-depth guide on how to create a Nandroid backup Method 2: Create a cloud backup. We have already covered this in an in-depth guide on how to create Android backup to the cloud
Have Developer Mode enabled: The last requirement is to have Developer Mode enabled on your device. This can be done by Navigating to Settings > Find your Build Number > Tap it 7 times until a popup appears