1. First of all, go here and download Nexus Root Toolkit 1.9.5. Install the software.
3. Make sure that your Nexus device has 'USB Debugging' option activated.
4. Launch Nexus Root Toolkit 1.9.5., press 'Flash Stock & Unroot), next press 'OK' and after this press 'Other'.. & 'I downloaded a factory image....'. Press 'OK' button and choose the Lollipop firmware image.
5. Watch the video for more details.
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