In case the system update is failing with a "Unable to determine size of system update" error message check first that your device has the correct date and time in "Settings > Time and date".
If the date/time was correct then please try the below procedure.
1) Connect your Sailfish device to the Internet
2) Connect battery charger
3) Enable Developer Mode - follow the instructions in this help article.
4) Open terminal app and give the following commands (you will need your SSH password for the 1st one from step 3):
ssu dr customer-jolla pkcon refresh version --dup
Commands "pkcon refresh" and "version --dup" will need some time to complete, please be patient through these commands.
Your device will restart with the "reboot" command.
Once your device has restarted, re-enter Terminal app and type in these commands to bring your device back to its normal state:
devel-su ssu er customer-jolla
You can now seek for OS updates in the regular way (Settings > Sailfish OS Updates).