Hi,

You can use odoo Post hooks for this problem:

You need to post_init_function in your manifest file:

              'post_init_hook':'add_field_test'

Import the post_init_hook function in your __init__.py file and define the function

from . hooks import add_field_test

Inside the hooks.py file define the function as follows,

from odoo import api,SUPERUSER_ID

def add_field_test(cr,registry):

env=api.Environment(cr,SUPERUSER_ID,{})

for rec in env[demo.model].search([]):

rec.field="value"

For more information refer: https://www.cybrosys.com/blog/how-to-use-hooks-in-odoo-development

Hope it helps