For old firmware sysupgrade without parameters is enough.
For newer firmware, use it as sysupgrade -k -r to update both kernel and rootfs.
If you got this error:
losetup: /tmp/rootfs.squashfs.${soc}: No such file or directory
Rootfs: Unable to get hostname, execution was interrupted...
then try to update only kernel first:
sysupgrade -k
If it doesn't help, use --force option:
sysupgrade -r --force
If you caught a glitch, retrieve the most recent version of the utility:
curl -k -L -o /usr/sbin/sysupgrade "https://raw.githubusercontent.com/OpenIPC/firmware/master/general/overlay/usr/sbin/sysupgrade"