Update Concrete5

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From the dashboard, Systems & Settings, Backup & Restore, Update concrete5 when I click on Update Concrete5 I get:
HTTP 500 error
That’s odd... the website can’t display this page
The site may be under maintenance or could have a programming error.
Try this
Go back to the last page
Try contacting the website’s owner

I am currently using:
# concrete5 Version
Core Version - 5.7.5.13
Version Installed - 5.7.5.13
Database Version - 20160615000000

View Replies:
A3020 replied on at Permalink Reply
A3020
I'd look in your server logs. Do you have access to those?
everythinghometv replied on at Permalink Reply
I get this error:
[Thu Dec 28 06:31:23 2017] [error] [client 66.249.79.157] Contact Zend Technologies athttp://www.zend.com/ for a later version of Zend Guard Loader.

Not sure how to upgrade this..