This was a bit tricky to find and doesn’t seem to be well documented.

    // version number below can be a number, i.e. 3 or a label
    let params = {
      Name: "/path/to/parameter:<version-number>",
      WithDecryption: false
    };

    ssm.getParameter(params, function (err, data) {
      if (err) console.error(err, err.stack);
      else console.log(data)
    });