I have an ESXi 5.0.0 server, free version, running at home. I have started fiddling with the command line tools recently and I found a neat one only available in ESXi 5.1. It's too cool to pass up, so I am going to try and upgrade my Server from 5.0 to 5.1.
I found this site http://www.vladan.fr/esxi-free-how-to-upgrade-to-esxi-5-1/  that contains the exact guide I want. I am going to record what I did in this post, pretty much following his guide.
Download the offline bundle
I need to download a file named VMware-ESXi-5.1.0-799733-depot.zip .
To do this you need an account at vmware.com (You don't need to pay for this account just register).
Head over to http://www.vmware.com/
Click On MyVMware
Click on Products then All Products.
Search for VMware-ESXi-5.1.0-799733
Hey that found it. Click on it to download.
I don't have permission to this file...
At  someone posted this
For those that are getting the error on VMWare’s site that says you don’t have permission: Click on the link to download the software, login, then click on the link again. You should now have access to download.
So I created a new account…
OK now I can download it.
Turn on SSH access
If I am going to do this via the command line I need access to the command line on the ESXi box itself. To do this I need to turn on ESXishell and SSH access.
Select the ESXi server
Open the Configurations tab.
Open Security Profile.
Click on Properties in the upper right.
This window should open. Select SSH. And click Options
Select "Start and stop with host" Then click OK.
Now select ESXi Shell and click Options.
Select "Start and stop with host" and click OK.
SSH and ESXi Shell are not running yet. But if you reboot your ESXi box they should come up automatically.
Click OK to close this window.
Right click on your box and select Reboot.
Give a reason, if you want, and click OK.
Wait for it to reboot.
Go back into Configurations -> Security Profile -> Properties
Now SSH and ESXi shell should be running.
Shut down all your running VMs
Copy the zip file over to the datastore
You can just use scp to copy the zip file over to the server.
(Replaces esxi with your ESXi IP address)
> scp VMware-ESXi-5.1.0-799733-depot.zip root@esxi:~/vmfs/volumes/datastore1/
SSH into the ESXi box
> ssh root@esxi
Run this command to update it.
> esxcli software profile update -d /vmfs/volumes/datastore1/VMware-ESXi-5.1.0-799733-depot.zip -p ESXi-5.1.0-799733-standard
After the command ran, scroll back a little and you, hopefully, will see this message.
Put the server in Maintenance Mode.
VSphere eventually booted me out. Then I just logged back in via VSphere.
Confirm it has updated
From the VSphere menu select Help -> About VMware vSphere
Looks like it worked!
Take it out of maintenance mode
Right click on the server and take it out of maintenance mode.
Start up some servers and confirm that they can work!
(Or in my case I just rebooted the server, since I have a Startup Order already set up.
Running a special command line test
> esxcli storage core device list | grep t10 | grep -v ":"
First run this to get a list of hard disks attached.
Now use this name to query the hard drive's status
> esxcli storage core device smart get -d t10.ATA_____WDC_WD40EZRX2D00SPEB0_________________________WD2DWCC4E7VRCL84
Looks like its running at 113 deg.
This feature was not in 5.0, but is in 5.1 J
I can see this drive can go up to 60°C = 140°F
 ESXi Free – how to upgrade to ESXi 5.1
 WD Green™ Desktop Hard Drives