Please understand I am not certain of which method you were using. The easiest method is to use the "UpdatePilot" App on your ST16S to obtain the latest update.
If UpdatePilot does not work for you, you can do a manual update using SD cards by following the instructions found in Attachment 31 (E90/H520 SD Card Firmware Upgrade) of the PDF document attached to
Way To Fix Drones project.
Note the following:
1.) These instructions will only work if you have the E90 camera.
2.) The link in the document leads to a source of two firmware versions. The Version from Late December caused problems for a lot of users, and most could only be resolved by sending the camera to Yuneec. At least one issue was reported after using the latest update.
There are no guarantees you will still have a functioning system after the updates. Regardless of how you perform the update, or where you get the files.
If your system is currently functioning normally and is doing everything you need it to do, you may want to consider leaving it as is until more confidence is gained in these latest updates.