Updates an existing form. This is a partial update — only the properties
present in the request body are changed; omitted properties keep their current value.

fields

Providing fields replaces the form's entire field set (same resolution rules as
POST /newsletter/{newsletterId}/form: email is auto-prepended for subscribe/order
forms, at least one field is required for update forms). Omit fields entirely to leave
the current fields untouched. To add or remove individual fields without replacing the
whole set, use POST .../fields / DELETE .../field/{fieldName} instead.

Order-form fields

shippingMethodIds, orderType, and products are a group for order forms
(method: "order"): if you provide one of them, provide all three — together they
replace the form's shipping methods, order type, and product list entirely.

method

The form's method (subscribe / update / order) cannot be changed after
creation. Sending a method value that differs from the form's current method
returns a validation error.

Limitations

• Only forms belonging to the authenticated account are accessible.
• Only non-archived forms can be updated.