This blog explains, step by step,  how to change the CMC admin user email when the current email address is no longer valid. 

Below is the command for how to change the email address of a CMC admin user:

The 'auth set recovery' command allows the admin password to be reset from the serial console for a Management server. The default is 'ON'.

If the option is enabled, you can type resetpass for the user name at the serial login and any non-empty password. Immediately after you log in, you will be asked to access the appliance. To verify access, disconnect the Network adapter, then reconnect it (see Note). This verifies that the serial console is not being accessed over a remote port replicator. If you do not have access to the appliance, you must set the recovery option to 'OFF'.

Follow this steps to reset the password if not known:
1. Go to Hypervisor for this image
2. Go to the console in Hypervisor
3. At the prompt where it asked for username enter "resetpass"
4. Enter a value for password (it cannot be an empty string).

Figure 1: Screenshot showing the result after step 4

5. After you log in, you will be asked to verify access to the appliance. Press the return key to proceed with the verification process

6. To verify access, you will be prompted to disconnect the Network adapter, then reconnect it.

To disconnect and reconnect the Network adapter, modify the adapter settings of the Management server. . Below is an example using the VMWare Hypervisor

This verifies that the serial console is not being accessed over a remote port replicator. If you do not have access to the appliance, you must set the recovery option to off.

More detail can be seen here:
https://www.ibm.com/support/knowledgecenter/en/SSMNED_5.0.0/com.ibm.apic.overview.doc/ref_CLI_auth_commands.html

Once you have reset the admin password, to change admin email address

1. Log in to the Cloud Manager user interface. using username 'admin' and password 'newpassword_you_set_above'

2. Click the User icon on top right hand coner then click My Account.

3. In the Details section, enter your new admin email address

4. Click Save.