Change the settings of the account

Only the fields you send change. billing_country, billing_address and intra_eu_vat_number clear when you send null; name, language and timezone keep their value on null, so name the language or the time zone you want. A postal address and a VAT number travel with billing_country: send the country in the same call, and the VAT number goes with an EU country. What you write applies to the invoices issued from then on. Writing the billing identity is owner only. Signing in is set up in the app: the email address, the password and the second factor are confirmed there by the person themselves.