Can I update from CM 12.1 to CM 13 using OTA, or do I need to follow instructions? I've got a Nexus 9 running Cyanogenmod 12.1. Yesterday I read Cyanogenmod's announcement of version 13, which contains instructions on how to update to the new version. They write that I have to flash the image, Google Apps and vendor.img manually.

However, today I received an update notification on my device:



Can I just apply the OTA update? Or do I still need to follow the instructions (i.e. flash Google Apps and vendor.img)?
The OTA-update will only update CM itself. You will still have to flash Google Apps and vendor.img manually. If you had Google Apps installed and only do an OTA-update, your Google Apps will most likely be incompatible with your system, causing your device to get stuck in a boot loop.

I don't have a Nexus 9, so I don't know if the old vendor.img is compatible with CM13, but I would recommend to update it as well, just to be sure.