When you select the storage device a box will appear stating that it's "Gathering Additional Information" - this is scanning for any previous installation of ESX / ESXi; once completed and it has deteceted your installation you will be given 3 options.
Force Migrate ESXi, Preserve VMFS Datastore: This will take your current installation and configuraiton and attempt an upgrade/migration and keep your VMFS Datastores intact, so your VMs will be preserved if things go wrong.
Install ESXi, Preserve VMFS Datastore: This will install a fresh copy of ESXi 5.0 and leave your VMFS datastores intact, this will erase all custom configurations, it's a clean slate basically and you will have to re-add your VMs back to your inventory once you have reconfigured your host server.
Install ESXi, Overwrite VMFS Datastore: This does exactly what it sounds, it will install a clean copy of ESXi 5.0 and overwrite your current datastores - you will LOOSE all information on that datastore.
For this Tutorial we are going to select Force Migrate ESXi because we are attempting an upgrade and we want our configuration to stay. Once you press enter the installation will scan for your configuration, once that is complete you will be given a confirmation warning, Be warned you can not roll back your installation to ESX or ESXi 4 - it is not supported! Press F11 to continue.
https://www.howtoforge.com/upgrade-m...-4-to-esxi-5.0