only have next button

something like this might work:

<?php if( get_next_posts_link() ) { next_posts_link('next'); } else { echo '<a href="' . home_url( '/' ) . '" title="home again">back to beginning</a>'; } ?>

http://codex.wordpress.org/Function_Reference/next_posts_link

replace your existing next/prev posts navigation with it;

the posts nav is often either somewhere after the start of the loop, or after the endwhile of the loop.

if you are not sure, look into other themes for inspiration.