If and how to upgrade mainly depends on your hardware (hosts, storage, ...) and whether it's supported for vSphere 5.5. So the first step is to verify this using the VMware Compatibility Guide.If everything is supported, the next step is to decide whether to upgrade vCenter Server or deploy a new instance (depends on used features, database version, OS version, ...). Once vCenter Server is upgraded or a new instance is installed and the hosts are connected to it, you can start upgrading or reinstalling the hosts, one at a time. This is usually a good time to also upgrade the host's firmware.
André