put https://public.billsby.com/api/v1/rest/core//customfields/
Update an existing custom field
You can update any existing custom fields using this API. You just need to provide us with the custom field ID and enter in the updates to the body parameters. You can update only the parameters you choose and the rest can be left blank. You cannot update the type of custom field; if you want to add a new custom field with the desired field type, you can use the Create custom field API instead.