to change data in the queries, use mutations

 Instead of query, use mutation.

query for reading data, mutation is for modifying data.

In the schema file, add below code.

const mutation = new GraphQLObjectType({

  name: 'Mutation',

  fields: {

    addUser: {

      type: UserType,

      args: {

        firstName: {type: new GraphQLNonNull(GraphQLString)},

        age: {type: new GraphQLNonNull(GraphQLInt)},

        companyId:{type: GraphQLString}

      },

      resolve(parentValue,{firstName,age}){

        return axios.post('http://localhost:3000/users',{firstName,age})

        .then(res=>res.data);

      }

    }

  }

});

module.exports = new GraphQLSchema({

  query: RootQuery,//created for queries

  mutation//mutation: mutation

});

Now, to run in graphQL UI, use the mutation keyword as below.

The exclamation symbols before fields in the docs section refers as mandatory fields.

//graphql

mutation {

  addUser(firstName: "Stephen",age: 26){

    id

    firstName

    age

  }

}