Current firmware version:
The quickest way to check the current firmware on any Samsung Android phone, dial *#1234#. On my phone I get:
This firmware corresponds to Android 2.3.3 (Gingerbread) with build date June 24, 2011. Note that 'PDA' value is the same as one see in Under Menu > Settings > About Phone > Build number (I have 'GINGERBREAD.XXKPE').
Install and run Samsung Kies  (Windows and OSX). Next, after rebooting the computer, connect the phone to the computer via a USB cable. Make sure to be connected to the internet. The Samsung Kies software will automatically detect and connect to the phone. If there is new firmware available, Kies will report this and offer to update the phone. Make sure to follow the instructions given. After a few minutes, the firmware is updated, and the phone will reboot.
New firmware version:
After updating via Samsung Kies, I get:
which corresponds to Android 2.3.3 (Gingerbread) with build date August 25, 2011.
PS. The reason why I updated in the first place was that my phone completely froze multiple times a day. The only thing to do when it happened was to restart the phone (reboot by holding Home and Power buttons for several seconds, or simply take out the battery). This is apparently a rather common problem. Update: After updating to KPS the phone has worked flawlessly for more than 10 days now.