How edit custom Configuration setting

I am new in dot net. I want edit the custom setting configuration parameters. Below is snap shot of App.config file and I want to edit destination and server parameters. How pragmatically I will do.

<ClientSettings><DestinationConfiguration><destinations><add NAME="HR_PROD" USER="hugo" PASSWD="password" CLIENT="000"/></destinations></DestinationConfiguration>/ClientSettings><ServerSettings><ServerConfiguration><servers><add NAME="HR_PROD_Elster" GWHOST="abap_server" GWSERV="sapgw21" REG_COUNT="5"/></servers></ServerConfiguration></ServerSettings>