Give it time
If for the most part your phone functions properly then simply giving the phone time can resolve your issues. This will allow application developers to update their applications or allow a patch to be written for possible bugs in the Android version itself.
Upgrade or remove the problematic applications
If there are specific apps or a singular application that no longer seems to run properly then you should check for updates regarding that specific application and if available update it. If there are no updates available then you could uninstall that application from the phone until a compatible version of that application is written for your version of Android and then re-download the app. Or you can try to uninstall and then reinstall the application to see if a fresh install will correct any issues that might be randomly creeping up.
Default apps or features are malfunctioning
If the applications that come preinstalled on the phone by default are crashing or malfunctioning such as your contacts, calendar, Play Store, Google processes, etc. or your phone is getting error messages regarding these functions or even if features you need don’t seem to be working properly then the BEST and surefire way to fix all these problems is to perform a full backup of all the information on the phone and then perform a factory data reset.
This will wipe everything from the phone and cleanup any software issues, hiccups, glitches, bugs, outdated apps, etc. that might be causing problems. The phone will think its brand new right out of the box. An important note to keep in mind though is that the phone will remain on the new updated software and will not go back to the previous version that that was installed on the phone before the upgrade.
After performing the reset I recommend that you use the phone without any custom or third party applications for a few days to make sure that everything is working like it should. This reset should resolve any issues that started after upgrading/updating the phone and chances are you might even notice an increase in performance and functionality on the device.