T42S unable to update firmware

[oliv]

I tried to update a T42S from 66.85.0.5 to 66.86.0.15 using web interface.
I followed the steps bellow:

1. download 66.86.0.15.rom file from Yealink web site
2. select downloaded file after clicking over Browse button
3. click over Update button
4. observe a message like Updating firmware over phone's screen

After 15 minutes, I pulled the phone patch cord and replugged it: it restarted with 66.85.0.5 version.

1. Which 66.86.0.x version shall I target first ?
2. Which 66.86.0.x version shall I target after the first one reached ?