querystring editmode

i'm passing an id on a querystring with a second parameter edit and i need to check if that edit mode is on or off butt frist i need to set it on codebehind:

PostBackUrl='<%# string.Format("~/admin_template/Insert.aspx?idp={0}&edit={1}", Eval("idp"),  secondparamhere) %>' so how can i set and change the second param in code behind

so it must be edit=y or =n but i need to set and change on code behind