Hi there, @CristinaValero ...
I am not sure if this will fully answer your question, but I thought I'd chime in. At our College, we have a "template" course (How do I enable a course as a course template?) where we have pre-defined our main grading scheme at the Canvas account level. Then, in our template course, we have set our College grading scheme in the course "Settings". Before a semester starts, when brand new course shells are created for the up-coming semester, that template course is copied out to all courses for the semester. The course includes some basic pages to help our instructors get started building content, but it also includes that setting for the College's grading scheme...so that it is the "default" for all courses.
I'm not sure if anything can be done from a programming aspect to update a grading scheme for courses that already exist. If that's possible, that is beyond my scope of knowledge.
Anyway, I hope this will be helpful in some way...for what it's worth. Hopefully someone else might be able to chime in if there are other back-end ways to change this for existing courses. Good luck!