connection string problem in winforms

Question

hi
can any one help me
how to change connections string at run time
current is working with first connection string. run time i need to change second connetion when first connection not found
<add key="ConnectionString1" value="server=server1;database=dbname;user id=sa;password=admin;"/>
      <add key="ConnectionString2" value="server=server2;database=dbname;user id=sa;password=admin;"/>