To remove the inscription «Proudly powered by WordPress», which is usually displayed at the bottom of each page, you need to edit the footer.php file of the active theme (for example, if the twentyfifteen theme is active, then /wp-content/themes/twentyfifteen/footer.php).
Namely, clear the content between the following tags:
<div class="site-info"> ...clear what's here... </div><!-- .site-info -->
Also noticed that if you use Jetpack with infinite scrolling, then it adds its footer, to open it you will open the file /wp-content/plugins/jetpack/modules/infinite-scroll/infinity.php and delete the line:
<?php echo $credits; ?>
The AMP plug-in’s footer is here – /wp-content/plugins/amp/templates/footer.php.
After updating the theme or Jetpack may have to repeat.