Thank you..at first it didnt work,after i googled,i found i missing <php code
here
<?php if( get_field('server_1') ): ?>
<button class="button" type="button" onClick="document.getElementById('frame').src='<?php the_field('server_1'); ?>'">Openload</button>
<center><iframe name="frame" id="frame" src="<?php the_field('server_1'); ?>" width="640" height="360" scrolling="no" frameborder="0"></iframe></center>
<?php endif; ?>