Hope this helps!
In addition to what @chofer wrote, if you're using the new gradebook, you might also check out: How do I override a student's final grade in the New Gradebook? Your Canvas Administrator will have to enable the feature for it to be available.
That option will allow you to change the grade without changing the underlying assignment grades.
Another technique is to add assignments worth 0 points and then insert positive numbers to raise their grades or negative numbers to lower their grades. How much to add or subtract and how to incorporate those depends on how your gradebook is set up.