Please reboot your phone and check for the update again. If you still can't receive it automatically, you can download the OTA file from here, and install it manually by following the instructions.
There is no OTA package for system downgrade. If you want to return to any previous ROM version you have to flash the complete ROM using SPFT tool. Don´t forget to flash everything, also I would advise to preform factory reset. When upgrading from Andoid 6 to Android 7 it is recommended to use OTA, because due to a bug you may lose your IMEI numbers whiile flashing Nougat ROM by SPFT.