How to set header from asyncData in NuxtJS

You can access asyncData if you you head() function.

Use 'head()' function in your script, not 'head:'

export default {
  head() {
    return {
      title: this.tag.name
    }
  },
  async asyncData({ $content, params }) {
    const tag = await $content('tags', params.slug).fetch()
    return {
      tag
    }
  }
}