Need Plugin to add Buttons

You mean you want to replace the text with buttons?

You should have something like this in your theme:

<div id="nav-global" class="navigation">
                        <div class="nav-previous">
                        <?php
                                next_posts_link('&laquo; Previous entries');
                                echo '&nbsp;';
                                previous_posts_link('Next entries &raquo;');
                        ?>
                        </div>
                </div>

Just change the next_posts_link and all 🙂

Try this: http://wordpress.org/extend/plugins/wp-page-numbers/