D2L doesn’t have a dedicated function for curving test or quiz scores, but there is a way to manipulate the grade item.

In the example below, the highest score was 65 out of 80. 

A fast way to curve this item would be to edit the grade item.

Then lower the Maximum Points value to 65.

There will be an "Are you sure?" confirmation message. Select Yes.

You will see that the maximum point value has been lowered, but student scores have stayed the same, effectively curving the grades.

NOTE: You should only curve the scores once you have recorded all the grades for that item. 

Credit to SRU.