Alex,
If what you want to do is to change the password of CIMC, then you need to restart the server, press F8 when you see the Cisco logo and then configure a new password (reason why you need to keep your servers away from ppl that can reboot your server and have access to it).
If by any chance you are looking to have the ESXi password changed, then you need to re-install the OS:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1317898
"Reinstalling the ESXi host is the only supported way to reset a password on ESXi. Any other method may lead to a host failure or an unsupported configuration due to the complex nature of the ESXi architecture. ESXi does not have a service console and as such traditional Linux methods of resetting a password, such as single-user mode do not apply. "
HTH,
-Kenny