I suggest you to download CNA from CCO.
This tool allows you to upgrade a 3K switch or stack by just point and clik/drag.
The .tar image contains a set of html files to be use with a tool called Device Mgr. this tool allows you to manage the switch. If you don't use that tool, then download a .bin file.
It is recommended to use the archive command since it will create the directory in flash and update the boot image name so the boot loader will find it quickly. Otherwise you have to do it manually.
The upgrade is not much different than upgradeing any Cisco router, tftp the image into flash and change the boot variable or just delete the older images from flash.
I hope this helps.
++Jose
