The customer can do this via their online account. They need to go to Account > Company details and click on the Edit company details option as indicated below:
Once done they will get a screen as below and will be able to update the details accordingly:
When the customer clicks on ‘UPDATE’, they will be shown a message asking if they would like to have previous invoices reissued with this updated VAT number (up to 3 previous invoices).
You will receive a Task on Teamhub to confirm these details.
In some countries, the customer may also be legally required to complete an additional addendum to confirm the changes to VAT. Please ask your CMTL or Ops Director for further local support with this if required, or raise an Ops Support request via TeamHub > Help.