Thank you sphereii for this nice tutorial !
You can add/modify an attribute with the setattribute command:
<setattribute xpath="[color="#FF0000"]PATH[/color]" name="[color="#FF0000"]ATTRIBUTE_NAME[/color]">[color="#FF0000"]VALUE[/color]</setattribute>
Example:
<setattribute xpath="/progression/attributes/attribute[@name='attPerception']" name="max_level">1000</setattribute>
It might be possible to remove an attribute with the remove command, but so far I got only NullReferenceExceptions with my tests.
Edit: the nullref is unavoidable using the remove command with an attribute.