If you want it to be done with a SQL request:

```
UPDATE mail_notification SET is_read = 't' where partner_id in (select partner_id from res_users WHERE login in ('admin', 'demo')) and is_read = 'f';
```

Where you replace `('admin', 'demo')` by the list of logins for which you want the notifications to be read.

If you want it to be done from a server action, using the ORM:

```
logins = ['admin', 'demo']
partner_ids = [user['partner_id'][0] for user in pool['res.users'].search_read(cr, uid, [('login', 'in', logins)], ['partner_id'], context=context)]
notifications_ids = pool['mail.notification'].search(cr, uid, [('partner_id', 'in', partner_ids), ('is_read', '=', False)], context=context)
pool['mail.notification'].write(cr, uid, notification_ids, {'is_read': True}, context=context)
```

Where you replace ['admin', 'demo'] by the list of logins for who you want the notifications to be read.