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<form name='yourForm' action="">
     <input type='text' name='yourinput' id='yourinput' value='Hello World'>
</form>

<script type="text/javascript"> 
// Assign the value to the input with the id passed in 
function replaceInputValue( idRef, val ) { 
	// Make sure browser supports getElementById  
	if(!document.getElementById ) return; 
	// Find the input by it's id 
	var inputObj = document.getElementById( idRef ); 
	if( inputObj ) { 
	// Update the value 
	inputObj.value = val; 
	}
}
replaceInputValue( 'yourinput', 'Hello Dolly' );
</script>