I came from 5.1-5055 to 5.2-5592... It has took me about 3 hours to find the correct procedure.
I tried to upgrade directly to the 5.2-5644 version of the 28.10.2015 but it didn't work for me.
I had to downgrade using the tip:
login to the console of your xpenology server
I replaced the 5644 version to 5565. I wrote the XPENOBOOT 5592.2 onto my USB stick with WIN32DiskImager
I rebooted my N54-L and choosed "Install/Upgrade"
I launched Synology Assistant and I think I have found what was wrong with my method:
I had to click on connect, not on install. This way, my browser opens, I choose installation , then migration, I point to the correct 5592 .pat file and so on...
The server reboots, and then, the synology splash screen appears... yeaaaah !!!
That's all. I was very frightened not to see back my samba shares and my volumes but finally it's OK !
For you, you just need to install the 5592 version on your key and follow my method (not really mine, of course) with synology assistant...
Hope this will be helpful for you !
I hope you will understand me, I'm french ;o)