Add:

"type": "module",

To the repository’s root level package.json file.

Now instead of

exports.handler = async (event, context, callback) => {

}

use this:

export async function handler(event, context) {

}

And any time you used

callback(null, {
  statusCode: 200,
  body: html,
})

Now use

return {
  statusCode: 200,
  body: html,
}