For classes where you want to give all Pass/Fail grades, select Clear Grading Scale and then modify the grading scale as described above. Define your new grade as P and assign its lowest value, for example, to be 60. Then define F with a value of 0. All students receiving 60 or above would receive a P and the rest an F.
If only some students are taking the class Pass/Fail, use the Multiple Grading Scales feature to set up both your main grading scale and a Pass/Fail system.