Sometimes the download of an OS update fails unexpectedly. Or else there might be the error message "Update size cannot be determined". This article has some advice how to recover from this situation.
1. Setting the scene
We recommend making at least some 3-4 GB of free space to the internal storage of your device before starting the OS download. Move your pictures, videos and other big files to your SD card (or to your PC or to cloud service). Check the space in "Settings > Storage". You should have 4 GB or more available.
Also, there are some apps from OpenRepos application distribution that typically cause problems when updating the OS. Please uninstall those apps before you proceed to update. Apps that have frequently caused problems are Phonebook, Callrecorder or Patchmanager, but there may be more of them.
Make sure the Internet connection you use is reliable.
Visit menu page "Settings > Sailfish OS updates" and try to get an update.
2. Eliminating possible conflicts
2.1 Unable to determine update size
Read this help article and do as requested in it.
2.2. Download starts but gets interrupted before complete
Sometimes the problem has turned out to be that the Store client (on the device) and Store backend (on Jolla internet) are not in sync with each other. We will try to eliminate some of those conflicts here.
We need to go to the command line, i.e. use Terminal application. You get this app by enabling the Developer mode (link to instructions). Remember to set the SSH password for yourself, too. Even a better tool would be to use the SSH connection from Putty app on PC - see this article.
Now, open either Terminal app or Putty terminal. Issue the following commands one-by-one followed by the Enter key:
devel-su ### you will need to type your SSH password here
rm -rf /home/nemo/.cache/store-client/os-info
rm -rf /home/.pk-zypp-dist-upgrade-cache/*
Your Sailfish device will restart now. Once up and running, visit menu page "Settings > Sailfish OS updates" and seek for updates. You should see the latest OS version available. Download and install it.
Best of luck!
3) Problem persists - collecting logs for Jolla R&D to investigate
In case OS download keeps failing, please run the following commands to collect detailed logs from your device.
devel-su ### (if not done already)
cp /var/log/systemboot.log /home/nemo
cp /var/log/systemupdate.log /home/nemo
cp /var/log/zypp/history /home/nemo
rpm -qa > /home/nemo/rpm-qa.log
export COLUMNS=400; journalctl -a -n10000 > /home/nemo/journal-out.log
ssu lr > /home/nemo/ssu-lr.log
tar -cvf Update-failure.tar systemboot.log systemupdate.log history rpm-qa.log journal-out.log ssu-lr.log
Then send file
Update-failure.tar from your phone to us, please.