how can i change connectionstring in application config file?

using Microsoft.Win32;

//Writing to the registry

string ConnectionStringValues = TextBoxServer; + TextBoxUID;...; 

 RegistryKey regkey = Registry.CurrentUser.CreateSubKey(@"Software\YourSoftwareName\RegistryKey");
regkey.SetValue("ConnectionStringName", "ConnectionStringValues");

//Reading from the registry (Getting your connectionString)
RegistryKey regkey = Registry.CurrentUser.CreateSubKey(@"Software\YourSoftwareName\RegistryKey");
regkey.GetValue("ConnectionStringName").ToString();

/*
Don't forget to create the registry key first in the registry
Open Run -> Type in the command regedit,
Expand HKEY_CURRENT_USER,
Locate and RightClick on Software folder,

New -> Key (Name it ConnectionStringName or any other name you prefer),
Right Click on the folder you created and choose: New ->String Value
You can leave the value field blank or initialize it with your current connection string

Also remember to include these registries when building your installer

Regards
*/

Please, read this: Connection Strings and Configuration Files[^]
LINQ to XML : Changing connectionString in app.config[^]

Related articles on CP site:
Saving Connection Strings to app.config[^]
How to Add/Change Connection String in App.config with Coding[^]