Upgrading Drupal

I successfully upgraded from 6 to 7 along with several subdomains on the same install. However, I forgot to follow the process for one domain. Here is the status of that domain:

The following messages appears in my website.

There is a security update available for your version of Drupal. To ensure the security of your server, you should update immediately! See the available updates page for more information and to install your missing updates.
There are security updates available for one or more of your modules or themes. To ensure the security of your server, you should update immediately! See the available updates page for more information and to install your missing updates.

Please help me to choose best version of drupal as I want to upgrade my previous version.

Thank you

Good day everybody!
I need to create discount based on product field value.
May be you advice me some ready to use module(which would be best) or other way to solve my problem.
Any help will be welcome!
It's realy very urgent - today is my birthday - but I had especially found time
Thank you!

I'm updating an old drupal 7 site.
I've first updated drupal core to 7.38.
I've now started to update contributed modules. I'm doing it by hand, one by one, running update.php after replacing the code in the modules folder;
c-tools (7.x-1.7)
views (7.x-3.11)
panels (7.x-3.5)
On update date (date-7.x-2.8, 4th module to update) I get the following;

I have the following fatal error when upgrading from 7.37 to 7.38.

Fatal error: require_once(): Failed opening required '/public_html/includes/theme.inc' (include_path='.:/usr/lib/php:/usr/local/lib/php') in common.inc on line 5195.

Here are my upgrade steps: