Recommended upgrade path from FortiOS 7.0.13 or earlier versions
If your device is running on version 7.0.13 or earlier, you must upgrade the device to version 7.0.14 first by following recommended upgrade path before upgrading to version 7.0.15 or later. Otherwise, the system can hang.
In the event of the system hanging due to the upgrade process to version 7.0.14, boot up from a backup partition using BIOS, then continue the upgrade by following the upgrade path.