Hello,
1. Download the required ESXi image.
2. Select the required host in the vCenter > Update Manager
3. Click Admin View and Click ESXi images
4. Import ESXi image and browse to the directory where this image is > Upload
5. The wizard asks you to provide a baseline name, give a baseline name (For example, host_upgrade)
6. Go back to Compliance view and Enter the host into maintenance mode. if the host is in maintenance mode, then no VM should reside on it. The host wont be removed from the cluster when it is in maintenance mode.
7. Click Attach and select only this created baseline containing the ESXi 5.5 ISO
8. Click Scan, the compliance status must come up as non compliant. If it comes up as incompatible then reboot that ESXi host and perform the scan again
9. Click Remediate and give a task name and keep the default values and finish the upgrade.
Couple of things here:
You need to make sure that your ESXi 5.5 is supported on the hardware. You can used Vmware Hardware Compatibility List to check that
vmware.com/go/hcl
Once the hardware is compatible for 5.5 ESXi, then you can proceed with the above upgrade.
Also, you need to put the host into maintenance mode during firmware upgrade.
Suhas