What would be the best solution to update the IOS on the whole cluster without a downtime?
Not possible. Here's the logic:
Let's say you are able to successfully push the firmware to the two units. If you reload only one of them, the one being reloaded will NOT be able to join the HA SSO pairing because of firmware incompatibility. \
I just finished an upgrade of three pairs of WiSM-2 HA AP SSO. What I've done is push the firmware across, force the APs to pre-download the firmware and then reboot the pair of WiSMs. The reboot is scheduled for 4 am.
Another alteration is this:
Let's say you have TWO pairs of WiSM-2 and both pairs are in AP SSO.
So you push the firmware to the two pairs. Next, you force the AP in one pair to go to the second pair. Reboot the first pair. For APs in the second pair, trigger the AP pre-download. Once the first pair of WLC come back online, move all the APs in the second pair to go to the first WLC. Once the APs have rebooted and joined the first WLC, reboot the second pair of WLC.