If your current SailfishOS is one of 0.99.5.11, 18.104.22.168 or 22.214.171.124, then the update to the latest version goes via the intermediate step of version 126.96.36.199.
1. Seek for updates in the usual way. The phone will offer an update to 188.8.131.52. It is recommended to use WLAN connection for the download and turn mobile data off. Check that your Jolla account really works by first installing an application from the Store, and if it succeeded, then download the OS update.
If the download of OS update gets stalled and/or you see the notification "Problem with Store" then check your Internet connection again. Then retry the download. Should the update keep failing then it is best to reset the phone to factory settings first and then try again. Read these instructions for factory reset, please.
2. As soon as 184.108.40.206 is installed to the phone, seek for another update. This time the phone will offer the latest SailfishOS version. Download and install it.
After the phone has rebooted, check that the update worked by checking the OS version in menu Settings > System > About Product. It has turned out that sometimes the update has failed and the phone has still version 220.127.116.11. In this case redo this step (update from 18.104.22.168 to the latest).