Tring to write to web.config file error Access to the path

Hi Friends.
Actually i m trying to make a setupfile for my project.That makes some processes and at the end of processes will change web.config Database connectionstring values and will run project.Evertthing is good but cant reach to web.config file beacuse of permissions.
Project will run on out of company hosting account.So i cant reach IIS to change permissions.

So please offer me a way or if you know solution.Please help.
I also thinking on downloading web.config file with ftp account and change it and reupload back with programaticly in code.
Waht you think???

ERROR:
System.Configuration.ConfigurationErrorsException: An error occurred loading a configuration file: Access to the path 'C:\Inetpub\vhos ...