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.
Please hit the "like" button if I've helped.