Upgrading packages may cause serious problems, including soft-bricking your device!
Blindly upgrading packages (manually or via script) can lead you into all sorts of trouble.
Just because there is an updated version of a given package does not mean it should be installed or that it will function properly. Ensure yourself before doing upgrade that it would be safe. Almost for sure avoid upgrading core packages.
There are two ways to manage and install packages in OpenWrt: with the LuCI web interface (System > Software), and via the command line interface (CLI). Both methods invoke the same opkg comand.
As of OpenWrt 19.07.0, the LuCI interface now has an 'Updates' tab with a listing of packages that have available upgrades. The LuCI Upgrade... button performs the same opkg upgrade command that is discussed in this article.
The same warnings apply to upgrading packages using LuCI and the CLI.
Generally speaking, the use of opkg upgrade is very highly discouraged. It should be avoided in almost all circumstances. In particular, bulk upgrading is very likely to result in major problems, but even upgrading individual packages may cause issues. It is also important to stress that this is distinctly different from the sysupgrade path for upgrading OpenWrt releases (major versions as well as maintenance upgrades). opkg upgrade will not update the OpenWrt version. Only sysupgrade can do that. The two are not equivalent.
Unlike the “big distros” of Linux, OpenWrt is optimized to run on systems with limited resources. This includes the opkg package manager, which does not have built-in ABI (Application Binary Interface) compatibility and kernel version dependencies verification. Although sometimes there may be no issues, there is no guarantee and the upgrade can result in various types of incompatibilities that can range from minor to severe, and it may be very difficult to troubleshoot. In addition, the opkg upgrade process will consume flash storage space. Since it does not (and cannot) overwrite the original (stored in ROM), it must store the upgraded packages in the r/w overlay.
In the vast majority of cases, any security patches of significant importance/risk will be rapidly released in an official stable maintenance release to be upgraded using the sysupgrade system. This is the recommended method for keeping up-to-date.
Those looking to be on the bleeding edge can consider using the snapshot releases, but should be mindful of the differences between stable and snapshot. Or, alternatively, build a custom image with the desired updated packages included in that image. The remaining users who still want to use opkg upgrade should only do so with selected individual packages (do not bulk update, and do not blindly update) and they should be aware that problems may occur that could necessitate a complete reset-to-defaults to resolve.
If you're already having issues, or wish to “undo” the upgraded packages: create a backup (optional; can be restored after the reset is complete) and then perform a reset to defaults (firstboot).
If you do choose to upgrade packages, especially with a script, you have been warned. Don't complain on the forum, and be ready to deal with the consequences, troubleshooting, and resolution yourself.