Save the connection in the Setting of C #

Good evening, thank you all in advance, I continue to pass my application from VB to C #, and with your help I can read an ini file, change the database in the chain to which I want to connect, now I have days trying to save the new string in the variable that I have in the setting of the C # project, I'm waiting for your help and I'll give you the code of what I need again.

In the project in configuration:

Nombre : mCnxSifco
Tipo   : Cadena de Conexion
Ambito : Aplicacion
Valor  : cadena a mi base de datos local

When I pass the new connection string, I want to replace or save the variable mCnxSifco with the new variable that contains the new connection called aoCnxSifco, but it sends me an error and I have not been able to save or replace it for my future connections.

Again I repeat that in visual basic I only wrote this instruction

Me.Item("mCnxSifco") = acCnxApp.Trim.Replace("qmasuser", acUserId)

I look forward to your help as I have been fighting or working on this code for weeks.

thanks

AM