What update did you download? Did it reboot your device? If so, it was *likely* an update to the OS, which can't be rolled back; I use a handset that's different than yours, so I'm not expecting any to mine and can only guess about yours. *IF* that's the case, your best bet would be to find a 3rd party calendar from Google Play and use that, as your default calendar app would be included as part of the OS update.
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