I would seriously check the device for virus, malware, and bad apps first. Should you find nothing, then you may want to seriously consider doing a "Hard Reset" on the phone. Obviously this isn't ideal as you will have to back up all personal files to another device or online account, and the apps will be removed, so you would have to get them again, however I see no other option, short of contacting the manufacturer. That I would only do if under warranty and if nothing else works. They in turn may well tell you to do the "Hard Reset" first.
This would remove any updates and restore the phone to its original state when you got it. In the future, I would really suggest not OKing any updates or changes that just show on your screen. In the case of a system update, I would close the option that shows up and go directly into the phone's "Settings" to make sure it in fact a legit update.
Also, sadly, sometimes they remove options when an update is made.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option.
6. When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
7. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
8. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
9. Then press the "Power" button to hard reset the phone.
10. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.