Hi Halim,
The best thing to do is to download all the ZD firmware (9.3, 9.5 adn 9.7) ahead of time onto a laptop, disconnect the ZD from the network plug your laptop directly to the ZD and upgrade them in this steps:
9.1>9.3
9.3>9.5
9.5>9.7
You can use the GA version of each of the major release, but you can review the release note on each to see the specifics.
Once you are in the final version of 9.7 (MR3 is the latest of the 9.7 code), you can plug the ZD back into the network and when the APs re-join the ZD they will upgrade their software once to the 9.7 version. As always make sure you take a backup at each steps just in case you need to restore it.
Sid