You can process your custom change using HOOK_form_alter
Example :
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
if($form_id == 'YOUR_NODE_FORM_ID') {
kint($form['field_name']['widget']); // show the value and field values
$form['field_name']['widget'][0]['value']['#default_value'] = 'My new value'; // for textfield.
}
}