Update per instructed in the phpbb board admin pages. Downloaded the update package, unzipped on local computer. Uploaded /install and /vendor to phpBB server and pointed browser to the install directory.

www.somesite.com is currently unable to handle this request.
HTTP ERROR 500
 
From the logs:
NOTICE: PHP message: PHP Recoverable fatal error:  Object of class Twig_Source could not be converted to string in /usr/local/www/somesite.com/forum/phpbb/template/twig/lexer.php on line 147

This is reported lots of times in the forums, but a reason/fix is not given afaik.