can't update Yuphoria to YOG4PAS47N (android 5.1.1), signature verification failed.

I have Yuphoria running android 5.0.2 (not rooted). I got this device as a replacement from micromax service center from Dadar (Mumbai). I have downloaded incremental update OTA that is YOG4PAS47N (android 5.1.1) but its not installing when trying to install. Message appears saying installing but after few secs it suddenly stops and shows the same install now screen. I have even downloaded sign zip & then tried installing via Cyanogen recovery again it failed saying "signature verification failed" . Then I downloaded full rom zip then tried but again signature failed message appears. Is there any other way to update to 5.1.1 (YOG4PAS47N). I have been googling from last 2 days but didn't find proper way to update. help me asap.

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