Set the element's disabled property to false:
document.getElementById('my-input-id').disabled = false;
If you're using jQuery, the equivalent would be:
$('#my-input-id').prop('disabled', false);
For several input fields, you may access them by class instead:
var inputs = document.getElementsByClassName('my-input-class');
for(var i = 0; i < inputs.length; i++) {
inputs[i].disabled = false;
}
Where document could be replaced with a form, for instance, to find only the elements inside that form. You could also use getElementsByTagName('input') to get all input elements. In your for iteration, you'd then have to check that inputs[i].type == 'text'.