Bought used nexus 6p and It isnt getting updates

So always bought moto x line for pretty quick updates but my moto x pure edition seems to have been forgotten by lenovo so I bought a Nexus 6p [always wanted to try nexus anyway} from craigs list. Well phone works great but ive noticed its last security patch was dec 5 2016 {its currently Feb 20 2017} and its on android 7.0. So shouldnt it be on android 7.1 and a lot newer security patch? in developer options the "allow bootloader to be unlocked isnt on. So any suggestions. Dont really wanna always side load. Never done that but Ive read once you do you never receive OTA updates anymore. If there is no other option ill do that if someone could kindly point me in the right direction. Dont really wanna brick my phone though lol. Any help would be appreciated. /s/Ronnie