To change the grading scheme in your course, click Grades in the course navbar.
Click the Schemes tab. The default scheme is set to percent by default. (You can choose which grade scheme your course will use by default when setting up the gradebook.)
To change the scheme from percent grades to letter grades, click on the check mark under the column header, "Set As Default."
To change back to the other scheme, repeat the steps you took to change the scheme but this time, select the other scheme.