You'll need to register for an account there to get downloading, so go ahead and do that.xda-developers is also a reputable source for device firmware.
Download and install the latest version from over here.
You may also need to download the Samsung USB drivers to your PC, also available at the same link.
With that, you're ready to start by putting your Samsung Galaxy S4 into download mode.
Now, that’s just the official way of installing updates.
If you’re not getting the update that way and are feeling adventurous, you can manually install a new version of Android on your Samsung Galaxy S4.
Keep in mind this sort of thing is pretty advanced.It's always prudent to back up your device data before starting.If you've unlocked the bootloader, you'll want to stay away from this method.First off, you'll need to figure out the specific model number for your Samsung Galaxy S4.Swipe down from the top of the screen of your phone to get the notification tray, tap the settings gear icon in the top-right. Down at the bottom, tap About device and make note of the model number.Now do a search for that model number at this site to find the latest available firmware for your device.