From your Start menu select “SharePoint 2010 Management Shell” or Start, “Microsoft SharePoint 2010 Products”, “SharePoint 2010 Management Shell”

Then enter:

  C:\PS> $passphrase = ConvertTo-SecureString -asPlainText -Force

  C:\PS> Set-SPPassPhrase -PassPhrase $passphrase –Confirm

 

After running these commends it aks for new passphrase value which a Farm admin account can set.