in this post, I'll show you how to change block title programmatically in Drupal 8.
Example
use Drupal\Core\Block\BlockPluginInterface;
function mymodule_block_view_alter(array &$build, BlockPluginInterface $block)
{
// This will uppercase the block title.
$build['#configuration']['label'] = mb_strtoupper($build['#configuration']['label']);
}
in this example I uppercase the block title.
