There are two types of curves that you can add to an assessment, both of which are found in the "Student Breakdown Options" section on an assessment page. One is "Award Percentage Points" and the other is "Change Points Possible". Both types of curves apply to all students on the assessment (even "Missing" students).
Be sure that you're in the "edit" mode when viewing an assessment if you want to apply or modify a curve.
Click the "Student Breakdown Options" bar to expand it and view your options.
Use the slider to adjust the curve for all students. When "Award Percentage Points" is selected, the slider will allow you to determine how many extra percentage points each student should get (up to a maximum of 100%).
In the screenshot below, notice how student raw grades and raw grade levels stay the same, but gradebook grades and grade levels adjust according to the extra percentage points awarded:
When "Change Points Possible" is selected, the slider bar allows you to adjust the overall number of points possible on the assessment. For this example, the assessment has two questions that are each worth 100 points, making the total number of points possible 200 (points possible are defined in the "Input" tab). Adjusting the points possible essentially changes the denominator used to calculate student scores.
In the example in the following screenshot, the number of points possible have been changed from 200 to 150. This means that students who originally had a 50% (or 100 out of 200 points) now have a 67% (100 out of 150 points):
It's also possible to add in a note that describes the purpose for applying a curve.
A couple of final notes: curves must be applied on an assessment-by-assessment basis – it's not possible to bulk curve multiple assessments all at once. It should also be noted that curves always apply to all students on the assessment – it's not possible to apply a curve to some students and not to others. If this needs to be done, it will be necessary to create two separate assessments: one for students that receive the curve, and one for students that do not.