First off, don't use INI files - they are seriously outdated! There are much simpler ways to do it now, particularly in .NET applications.
Open your project in Visual Studio, and look at the Solution Explorer pane. Open teh project bach, and double click on "Settings.settings"
In the page that appears, add a new setting:
Name: ConnectionString
Type: String
Scope: User
Value: --Set to your default connection string--
Save the file (CTRL+S will do it) and close the page.
Now, in your code you can just say:
string strConnect = Properties.Settings.Default.ConnectionString;
and the system will sort it all out for you.
If you want to change it, just reverse the process:
Properties.Settings.Default.ConnectionString = strConnect;
Properties.Settings.Default.Save();