Check to see if all your system apps are up to date.
After that perform a soft reset - hold volume down and the power button together until the phone vibrates. The phone will restart. No personal data is lost.
Check for updates.
If that doesn't work you have two options.
1 - hard reset. Settings - About - Reset phone
This will erase everything on the phone so be sure to backup what you can. Keep in mind that any app that uses in app data may not be backed up unless it has this option. That includes games.
You need a PC for this with an internet connection. This will download and install the latest firmware onto your phone. As with the hard reset this erases everything on you phone.