To modifying a repor in Odoo-9 follow the below steps one by one:
Step-1 First we have to Modify existing reports which will then replace the originals in our OpenERP database,
Step-2 create new reports for the selected object.
Step-3 select Report > Modify Existing Report . Choose the report Request for Quotation in the Modify Existing Report box and then click Save to Drive .
Step-4 And change the design code part of initial module.
For example code is below.

{
    "class1": "foo",
    "title": "Random Title",
    "items": [
        { "name": "foo", "tags": {"bar": "baz", "qux": "quux"} },
        { "name": "Lorem", "tags": {
                "ipsum": "dolor",
                "sit": "amet",
                "consectetur": "adipiscing",
                "elit": "Sed",
                "hendrerit": "ullamcorper",
                "ante": "id",
                "vestibulum": "Lorem",
                "ipsum": "dolor",
                "sit": "amet"
            }
        }
    ]
}