If your Samsung device displays strange behaviour after a firmware update, DON'T PANIC !
There is nothing wrong with your phone.
What's happened is that some of the old system files have become corrupted by the new system files and you need to "reset" the phone to clear the conflicts.
This process will delete everything from your phone (except anything on your sim card or external SD card) and will reset the operating system so your phone runs smoothly again.
You will still be on the latest updates (the one you just downloaded) as you cannot roll back this way.
First you need to back up your data, unless you're not worried about losing it.
For older Samsung devices; Feature phone, Smart Phone/Tablet before Galaxy Note III (including Galaxy S4, Note II, etc) you need to download Kies to your PC or laptop Kies and carry out a full back up. Instructions are on the link.
For newer devices, Smart Phone/Tablet from Galaxy Note III (Android OS 4.3 or higher) you need Samsung Smartswitch. We have a seperate guide for that here Smartswitch
After this, go into Settings >back up & restore and make sure the options to back up & restore device are ticked.
Next, make sure you are connected to wifi and reboot the phone, wait 5 mins before turning off again (this allows the phone to back up apps to Google servers).
With phone off, hold Vol Up + Home + Power all together until the phone boots up, then let go. (For Galaxy S8 and S9 hold the Volume up, Bixby, and Power buttons at the same time, and keep them held down until you see the Samsung logo).
You will now be in the bootloader menu.
Use vol down to select the option to reset all user data (or similar words) and when highlighted, press the power key.
Wait for menu to appear again and then press the power key when "reboot" is highlighted.
Phone will now restart and prompt you to re-enter your google account and set up some of the defaults on the phone.
Once it fully boots up your apps will automatically download again to the phone.
After all the apps have downloaded, reconnect to Kies and restore the backup you made earlier.
Simples !!