How to upgrade from ESX 3.5 to ESX 4.1

U cannot upgrade an ESX server From ESX to ESXi

In order to upgrade an ESX server from Version 3.5 to Version 4.1, u need to perform 2 steps to achieve this.

Step 1: Upgrade the ESX Server from Version 3.5 to ESX 4.0 then
Step 2: Upgrade the ESX Server from Version 4.0 to ESX4.1

For First Step we can use VMware Host Update Utility which comes with VMware Vsphere Client 4.0

Pre-requisites:

VMware Vsphere Client 4.0 with Host Update Utility
VMware Vsphere ESX 4.0 ISO image

Action Steps:

1. Install VMware Vsphere Client 4.0 on Vcenter server or any windows server that have access to your ESX server. While installing please check the vmware host update utility check box to install the same.

2. Store the ESX 4.0 ISO image on Vcenter server or any windows server.

3. Shutdown or move the virtual machines running on that ESX host to another ESX host.

4. Bring the ESX server to maintenance mode

5. Launch the VMware Host Update Utility --> click on Add new Host --> enter the IP address or domain name --> Click on Upgrade Host  --> Browse the ISO image then click on Next -->Accept the License Agreement --> Provide the credentials of the server to upload the iso image to ESX Server -->

6. Click on Finish to upgrade the Server