Ok so I will give some direction and link you to a page.
First let me ask you this the connection you are using presently is it in a web.config file. If so this is where you will change the server/instance and database/catalog name.
Web.Config modification for SQL Server Instance
or this link
Web Config modification for SQL Express 2
So if you in the web.config file you should see something similar to this.
<ConnectionStrings>
<add name="" connectionString="Server=(localhost)\SqlExpress;Max Pool Size=300;Initial Catalog=database;User ID=username;Password=password;" providerName="System.Data.SqlClient"/>
</Connectionstrings>
or
<connectionStrings>
<add name="DefaultConnection" connectionString="Data Source= (LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\database.mdf;Initial Catalog=database;Integrated Security=True" providerName="System.Data.SqlClient" />
you need to change the follow:
Server or Data Source
and
Initial Catalog or AttachDbFilename
Now your credentials you may use any username and password that has access to the sql server express instance.
I hope this helps.