If you open your FTP software and go in here /packages/theme_slate/themes/slate/
you might find a "footer.php" file (often in a "elements" directory). In this file you'll probably find the footer you are trying to modify.

Thanks...i've found it now.

Which lines can I delete?

Thanks for your feedback!

[
<?php defined('C5_EXECUTE') or die(_("Access Denied.")); ?>
<footer>
<div class="row">
<div class="col_3">
<?php
$a = new GlobalArea('Footer');
$a->display();
?>
</div><!-- .col_3 ends -->
<div class="col_3">
<?php
$a = new GlobalArea('Footer Column Two');
$a->display();
?>
</div><!-- .col_3 ends -->
<div class="col_3">
<?php
$a = new GlobalArea('Footer Column Three');
$a->display();
?>
</div><!-- .col_3 ends -->
<div class="col_3 last">
<?php
$a = new GlobalArea('Footer Column Four');
$a->display();
?>
</div><!-- .col_3 ends -->
</div><!-- .row ends -->
<div class="row">
<div id="credits" class="col_12 clearboth">
<p class="left">© <?php echo date('Y')?> <?php echo SITE?>. <?php echo t('All Rights Reserved.')?> <?php
$u = new User();
if ($u->isRegistered()) { ?>
<span class="sign-in"><?php echo t('Currently logged in as <span class="user"><b>%s</b></span>.', $u->getUserName())?> <a href="<?php echo $this->url('/login', 'logout')?>"><?php echo t('Sign Out')?></a></span>
<?php } else { ?>
<span class="sign-in"><a href="<?php echo $this->url('/login')?>"><?php echo t('Login')?></a>.</span>
<?php } ?></p>
<p class="right"><span class="star"><?php echo t('Design by')?> <a href="http://www.c5mix.com" title="<?php echo t('concrete5 themes')?>"><?php echo t('c5mix')?></a></span>   |   <span class="settings"><?php echo t('Powered by')?> <a href="http://www.concrete5.org" title="<?php echo t('concrete5 CMS') ?>"><?php echo t('concrete5') ?></a></span></p>
</div><!-- #credits ends -->
</div><!-- .row ends -->
</footer><!-- footer ends -->

</div><!-- #container ends -->

<?php Loader::element('footer_required'); ?>

</body>
</html>)
]