How to uncheck a radio button?

you can access form like so ...

var exampleForm = document.forms['form_name'];

then loop through the form

for( var i=0; i<exampleForm.length; i++ ){
   alert( exampleForm[i].type );
}

you can test for checked like so ...

if( exampleForm[i].checked ) 

to deselect the checked radio button try ...

exampleForm[i].checked=false;

the final code would look like this ...

var exampleForm = document.forms['form_name'];
for( var i=0; i<exampleForm.length; i++ ){
   if( exampleForm[i].type ) == 'radio' && exampleForm[i].checked == true ){
       exampleForm[i].checked = false;
   }
}

I'm not sure exactly what you want but you might try using a reset input.

<input type='reset' />