unable to install an update


AC Question

I have Micromax Canvas A1(aq4501), running marshmallow 6.0.1(December 2015 security patch) Android one device.
Problem is that whenever I turn on my phone it shows Micromax logo with oem unlocked(I don't know how it is happened) my phone is not rooted and it's completely clean,Now, when my phone is getting ota update it's downloaded but during installation it shows error,i have try it many times and every time it is failed.