Cannot get prestashop to upgrade

[artflow]

I don't understand how to upgrade prestasop. I have version 1.1.0.5 working, but whenever I upgrade to 1.2.x and try the new version I get this:

Warning: Invalid argument supplied for foreach() in /home/artflow/public_html/prestashop/classes/Tools.php on line 835

Warning: date_default_timezone_set() expects parameter 1 to be string, array given in /home/artflow/public_html/prestashop/config/config.inc.php on line 155

Warning: Cannot modify header information - headers already sent by (output started at /home/artflow/public_html/prestashop/classes/Tools.php:835) in /home/artflow/public_html/prestashop/header.php on line 4

Warning: Cannot modify header information - headers already sent by (output started at /home/artflow/public_html/prestashop/classes/Tools.php:835) in /home/artflow/public_html/prestashop/classes/Cookie.php on line 261

Warning: Cannot modify header information - headers already sent by (output started at /home/artflow/public_html/prestashop/classes/Tools.php:835) in /home/artflow/public_html/prestashop/classes/Cookie.php on line 261

Fatal error: Declaration of SEKeywords::getData() must be compatible with that of ModuleGraph::getData() in /home/artflow/public_html/prestashop/modules/sekeywords/sekeywords.php on line 13

I've upgraded as per the instructions on the download page, many times over, but it just won't work.

-mats