First, shoutout to ashraf aaref, who's answer I would like to expand a little.
As MDN Web Docs suggest, using RadioNodeList is the preferred way to go:
// Get the form
const form = document.forms[0];
// Get the form's radio buttons
const radios = form.elements['color'];
// You can also easily get the selected value
console.log(radios.value);
// Set the "red" option as the value, i.e. select it
radios.value = 'red';
One might however also select the form via querySelector, which works fine too:
const form = document.querySelector('form[name="somename"]')
However, selecting the radios directly will not work, because it returns a simple NodeList.
document.querySelectorAll('input[name="color"]')
// Returns: NodeList [ input, input ]
While selecting the form first returns a RadioNodeList
document.forms[0].elements['color']
// document.forms[0].color # Shortcut variant
// document.forms[0].elements['complex[naming]'] # Note: shortcuts do not work well with complex field names, thus `elements` for a more programmatic aproach
// Returns: RadioNodeList { 0: input, 1: input, value: "red", length: 2 }
This is why you have to select the form first and then call the elements Method. Aside from all the input Nodes, the RadioNodeList also includes a property value, which enables this simple manipulation.
Reference: https://developer.mozilla.org/en-US/docs/Web/API/RadioNodeList/value