I have already answered the same question in different thread. Take a look here:
Sysupgrade is not present in Turris OS 3.x release and will be removed in Turris OS 5.0. It is dangerous and it shouldn’t be used as Turris routers are automatically updated.
If you would like to have Turris OS 5.x, which is based on top of OpenWrt 19.07, I suggest to take a look in this thread: