Need some help on how to update the BIOS

Hey guys,i need some help again. my MOBO a Asus P5KPL (really old MOBO,im upgrading to a z77) needs a bios update,the main reason is the fact that first,i havent updated the BIOS for quite some time (4 years to be precise) and second all of a sudden my windows 7 64 bit is reporting that only 3.50gb out of 4gb is usable,and the reason for that seems to be the outdated bios. however i never updated a bios before,and im really scared that i might brick the entire motherboard even though it supports crashfree bios,so what should i do? i already got the new bios and the update tool from the asus website, which is the Afudos BIOS update tool V2.36,however im not sure what to do with it,could you guys lend me hand?

It's not unusual for Windows to report less RAM than is physically installed, whether 64-bit or 32-bit.  The reason for this is that the OS has to map other things like device drivers and video graphics space to the same memory space.  The biggest culprit is generally video.  In your case, with 4GB of physical memory approx 512MB is being mapped to video and other such things.  As your os is 64-bit if you add more memory, say another 4GB, then that entire extra memory would be available to Windows, ie 3.5GB + 4GB = 7.5GB.

As for updating the BIOS, visit the ASUS web-site and they should have instructions there.  I wouldn't update the BIOS for the sake of it, but seeing that you haven't updated it for 4 years it's probably not a bad idea.  Follow the instructions provided by the mobo manufacturer. 

By the way, there is always a chance of 'bricking' the motherboard when updating the BIOS, for example if the power should fail during the update.

thank you for the answer,but doesnt windows only map the 512mb for video,only if you have a integrated gfx card? or it does the same thing with non integrated gfx cards? (a HD 4850 here)