I just added a block to the 'content' region. It shows up at the bottom of this region. How do I do to move it to the top of this region?
I'm using the Drupal default theme: Garland
If it's showing up below the page content (and you want it above), you'll need to add a new region. It's recommended to copy Garland and rename it as your own custom theme in sites/all/themes.
In the .info file, add a new region. e.g.:
regions[content_upper] = "content upper"
In the page template file, place that region just above $content.
<?php print $content_upper; ?> <?php print $content; ?>
Then place the block in "content upper"