FIND:

            <span class="nav-previous"><?php previous_post_link( '%link' , __('', 'cosmotheme') ); ?></span>
            <span class="nav-next"><?php next_post_link( '%link' , __('', 'cosmotheme') ); ?></span>

EDIT IN:

            <span class="nav-previous"><?php previous_post_link( '%link' , __('', 'cosmotheme'), $taxonomy = 'category' ); ?></span>
            <span class="nav-next"><?php next_post_link( '%link' , __('', 'cosmotheme'), $taxonomy = 'category' ); ?></span>