How alter user's database on connectionString in runtime without modify web.config

My question is this: do I have to after the login page of my web application, just change the user's database on connectionString, but without changing anything in web.config?

Thank you

Not sure what you mean?

The connection string can be picked up from the web.config file, it can be hard coded, it can be modified at runtime. It all depends on how you are using it.