Question

How to modify the data of a domain's contact handle (owner/admin/tech)?

Answer

In order to update data of a contact handle (e.g. email address, phone number, address, VAT number, etc.) one should navigate to Customer management > Customers overview > XX000001-XX in Reseller Control Panel:

Then click on the Edit customer button and change information in the available fields:

Additional information required

The registry decides what information is required in order to successfully register (or transfer) a domain. Examples can be the presence of a VAT number, a social security number, a passport number, state/province in the address details, etc. As this information is not required for all registries, those fields are also not mandatory when one creates a handle. In case one requests a domain that requires those extra details, an error will appear that "additional information or data is required". This can be added to the existing customer handle.

Which details required for a certain extension can be found on the detail page of that extension.

How to update the Company Name, First Name or Last Name

Not all data is editable; the first name, last name, and company name can never be changed once a handle is created. For some cases, the reason behind this is a registry restriction, for others - legal limitations.
Example: The information at some registries will not be updated (only allowed via a trade or documents) and this would cause a mismatch in details in the control panel and in the registry (actual domain) details. Therefore, it is not possible to adjust the company name, first name, and last name of the handle to prevent this.

Tip: One can Clone a customer in order to get a pre-filled new customer, where updates can be made. This will create a new handle that can be used to update the domain.

You can find the clone button as shown below: