1) Download the depot file; at the time of writing the latest update is as follows and can be found here.
VMware-ESXi-5.1.0-799733-depot.zip
File size:298M
File type: zip
Release Date:2012-09-10
Build Number:799733
2) Upload the file to your datastore, in this example the file was uploaded to “myDatastore”
3) Enable SSH on your ESXi host and on the command line, enter the following, using an absolute path to your depot file:
~ # esxcli software profile install -d /vmfs/volumes/myDatastore/VMware-ESXi-5.1.0-799733-depot.zip -p ESXi-5.1.0-799733-standard4) Reboot, done!