You couldn't have, one of a couple things must have happened:
The USB stick didn't actually get written with 2.5.0 for one reason or another
Your disk, if it's an SSD or similar media, has failed into a read-only mode and the writes from the installation didn't actually happen on the disk, so it's still running the previous version.
The second reason could also explain why the upgrade failed.
There are other potential causes but they all center around the hardware in the unit, not the installation image itself.