Hi JJ...I highly recommend looking at the Upgrade Center at VMware's site:
http://www.vmware.com/products/vsphere/upgrade-center/resources.htmlTypically...VMware upgrades are fairly straight forward, but you obviously want to take the proper precaustions. You should back up your vCenter DB before upgrading, as well as make sure you have a current b/u of all your VMs. Once that is done, just follow the upgrade videos or the upgrade guide in the URL I provided.
http://www.vmware.com/pdf/vsphere4/r40/vsp_40_upgrade_guide.pdfRegards,
~coolsport00