how to update the update a document in faunadb code example

Example 1: faunadb update

client.query(
  q.Update(
    q.Ref(q.Collection('spells'), '181388642581742080'),
    {
      data: {
        name: 'Mountain\'s Thunder',
        cost: null,
      },
    },
  )
)
.then((ret) => console.log(ret))

Example 2: faunadb update

Update( ref, param_object )