Provided that the email address is part of your current VM broadband account follow these instructions:
Sign in to My Virgin Media, go to My Profile and/or Manage Accounts depending on whether the account concerned is your primary account or an additional one. Select the account concerned and then click on Change to the right of password if a primary account or View/Change then change to the right of password for secondary accounts.
If not then post back here with more details.
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