i use Zorin 17.3 and i followed the instructions from post [HOW TO] set an older kernel to default boot - #11 by Aravisian
no idea why
sudo apt reinstall linux-headers-generic linux-image-generic did installed me an kernel 5.... ????
and under reinstall i mean sudo apt reinstall sorry 
and yes software update is set to main server and i downloaded Zorin form official web site here at the same start from Zorin os 17