Method 1:
Login to the console as root (with the root password you know) and execute “passwd”.
Method 2:
– Start your VMware Infrastructure Client, pointing to the ESX server.
– Log in
– Click “Users & Groups”
– Double click “root”
– click the “Change password” checkbox
– type the new password in the “Password” field and in the “Confirm” field, and click “OK”
Method 3:
If you don’t know the root password anymore, see: http://virtual.bist.be/?p=101 for a procedure (see also http://kb.vmware.com/kb/1317898). Not difficult, but you’ll have to bring the server down first.