Upgrading all packages on OpenWrt with the opkg command isn’t possible. There is no opkg upgrade --all command or anything similar. opkg upgrade expects a package as the first parameter that then gets upgraded. But with a little bash scripting we can easily implement the behavior we want (upgrade all the packges).

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for pkg in `opkg list-upgradable | cut -d' ' -f1` | grep -v Multiple; do opkg upgrade $pkg; done

It wouldn’t advise doing an upgrade like this. The best way is still to individually upgrade each package and read the upgrade notes.