Several tools are available to upgrade and migrate hosts. You can use different upgrade tools based depending on the type of host you are upgrading (ESX or ESXi) and whether the hosts are managed by vCenter Server.
You can migrate or upgrade to ESXi 5.x from version 4.x ESX or ESXi or version 5.0.x with the tools and methods described in ESXi 5.5 Upgrade Options.
To upgrade version 3.5 ESX or ESXi to ESXi 5.x, you must first upgrade version 3.5 ESX or ESXi to version 4.x ESX or ESXi. See the VMware vSphere 4.x documentation Web page for information about upgrading from version 3.5 ESX or ESXi 3.5 to version 4.x ESX or ESXi.
If you upgrade hosts managed by vCenter Server, you must upgrade to vCenter Server before you upgrade ESX or ESXi. If you do not upgrade in the correct order, you can lose data and lose access to your servers.