To update all installed packages on a Gentoo Linux system, you can use the emerge command with the --update flag.
Here is the general syntax for updating all installed packages with emerge:
emerge --update --deep --newuse @world
This command will update all installed packages and their dependencies, as well as any packages that depend on them. It will also update any packages that have new USE flags enabled or disabled.
If you want to update only a specific package or group of packages, you can specify the package names after the @world keyword. For example, to update the firefox package and its dependencies, you can use the following command:
emerge --update --deep --newuse firefox