To change the passing criteria in a course test paper you will need to login as the Administrator. Each course has a default pass setting of 80%.
Go to Configure Courses and from the dropdown menu select Change Pass Percentage; this will open up a new page where you will see the current pass mark percentage.
You can increase or decrease this, when you have your new passing value, click udpate; the next time the test paper is taken, the new percentage will come into force.