Changing the letter grade parameters depends on the Moodle version you are in and how your site has been set up. Some Administrators set this up at site level and do not allow changes at the course level. Here is some of the documentation for 3.2. If you are in a different version, you can change the 32 in the web address to the Moodle version you are using to see the page as it would apply to your version.
If this doesn't work for you, you can possibly change the calculations in the course using categories etc. Here is some information.
Hope that helps