If you lose the [email protected] password then it is possible to reset it using the following process.

1. SSH to the Vcenter appliance as root
2. Enter "SHELL" to start the bash shell
3. enter "sudo /usr/lib/vmware-vmdir/bin/vdcadmintool"
4. The following menu is displayed: 
5. Select 3. Reset account password
6. A generated password will then be displayed

It is also possible to change the password using the vSphere Web Client.

1. Go to Menu -> Administration -> Single Sign-On -> Users and Groups -> Users.
2. Select the Domain as "vsphere.local".
3. On entry for Administrator select the 3 dots menu and then "Edit"
4. Enter the new "Password" and "confirm Password" then "Save"

Log off and back on to test.

More info on the VMWare Knowledge Base: https://kb.vmware.com/s/article/2061122