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):

devel-su
ssu dr customer-jolla
pkcon refresh 
version --dup
reboot

Commands "pkcon refresh" and "version --dup" will need some time to complete, please be patient through these commands. The latter command seeks for potential missing pieces and then installs them.

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).