I had a try updating-upgrading the distribution with some errors:
pi@navio:~ $ sudo apt-get update && sudo apt-get dist-upgrade
Get:1 Index of /debian stretch InRelease [25.4 kB]
Get:2 Index of /raspbian stretch InRelease [15.0 kB]
Hit:3 http://files.emlid.com/navio2/debian stretch InRelease
Get:4 http://packages.ros.org/ros/ubuntu xenial InRelease [4,678 B]
Get:5 Index of /raspbian stretch/main armhf Packages [11.7 MB]
Get:6 Index of /debian stretch/main armhf Packages [221 kB]
Err:4 http://packages.ros.org/ros/ubuntu xenial InRelease
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
Fetched 11.9 MB in 23s (513 kB/s)
Reading package lists… Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.ros.org/ros/ubuntu xenial InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
W: Failed to fetch http://packages.ros.org/ros/ubuntu/dists/xenial/InRelease The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
W: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists… Done
Building dependency tree
Reading state information… Done
Calculating upgrade… Done
The following package was automatically installed and is no longer required:
libbluray1
Use ‘sudo apt autoremove’ to remove it.
The following packages will be upgraded:
libraspberrypi-bin libraspberrypi-dev libraspberrypi-doc libraspberrypi0 patch raspberrypi-bootloader
6 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 36.7 MB of archives.
After this operation, 84.0 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y