Hello,
if you have Turris OS 4.0.5, just use in CLI this command switch-branch hbk and you will have OpenWrt 19.07.xx. But as described in our documentation HBK is for experienced users.
Turris OS 4.0.5 which is based on top of OpenWrt 18.06 is still supported and the EoL is, for now, scheduled to the next month and by the time and even sooner it will be in the stable branch.
You haven’t told us, why you would like to try OpenWrt 19.07.02 on your router. You shouldn’t use sysupgrade or flashing router via LuCI. That’s not supported and will be removed. And if anything goes wrong, it is not supported. It might require the serial console or sending to us.