How to update a single field in a Contentful entry?

Need help updating a field in a CF entry. It is a dropdown field without a default value.

I tried to do that by getting the entry from the CF environment, setting the corresponding field and updating it like this:

        client.getSpace(spaceId)
        .then((space) => {space.getEnvironment(environment)
            .then((environment) => {environment.getEntry('1234567890')
                .then((entry) => {
                    entry.fields = {
                        state: {
                            'en-US': 'Arizona'
                        }
                    }
                    return entry.update()
                })
            })
        })

By doing so, the state value gets updated, but the other existing field values are deleted.

The Contentful documentation for content update states the same here: https://www.contentful.com/developers/docs/references/content-management-api/#/introduction/updating-content, but I cannot find a way to implement their suggestion.

How do I update the state value without losing all the other fields?