ok since ive never done that which is best factory image or OTA file....and which is safest?
The OTA doesn't require unlocking the bootloader and everything you do via the OTA file can be overridden by flashing a factory image. The OTA is basically going to download the whole thing and install it as a ROM update on your device, which means it is going to try to leave your apps, data and settings intact and just upgrade the firmware. The factory image is like a new clean slate. For this, OTA should be fine if you use the full OTA images from the Android Developer's site here:
https://developers.google.com/android/ota
High level process:
On your phone: you turn developer settings on by tapping the build number 7-100 times until it says you're a developer, turn on adb inside dev settings, go to security settings, enable oem unlock (which you won't be doing, but do this JUST IN CASE we have to go the factory image route)
On your PC:download the image from that site above, download the SDK so that you have adb tools on a PC, put the files in a place that makes sense, directions are on the site above ^^
plug the phone into the PC, go to the notification that says connected and charging or whatever, change that to FTP or file transfer, then you can type
fastboot reboot recovery to get into recovery, then hold the volume up key and hit the power key (reverse that if it doesn't work) and you'll get a menu, use the volume down key to navigate to apply update via adb sideload or something that looks like that, hit power to select it. Then on your PC you can type
adb sideload insertfilenamehere.zip and it'll do some stuff, when it's done, you hit power to reboot and you're done.
If that all sounds rediculous, we can help you find some guides and if it can wait a few days I could probably find time to help walk you through it on Skype or something similar. Thursday evening would be my earliest though.