Hi,
You can find here a documentation for upgrade your ESXi. http://www.vmware.com/pdf/vsphere4/r41/vsp_41_upgrade_guide.pdf
But major step are :
- 1 Check if your host are compatible 4.1 using HCL. VMware Compatibility Guide: System Search
- 2 Upgrade your vCenter to vCenter 4.1
- 3 Put one host to maintenance mode.
- 4 Upgrade this host using update manager or using update ISO. VMware vSphere 4: Private Cloud Computing, Server and Data Center Virtualization
- 5 Repeat 3-4 for all your host.
regards,
Julien.