How to save excel in specific path in ASP.NET using C#

System.Threading.Thread.Sleep(3000);
           System.IO.StringWriter sw = new System.IO.StringWriter();
           HtmlTextWriter htw = new HtmlTextWriter(sw);
           gvEmpdetails.AllowPaging = false;
           BindGrid();
           //Applying stlye to gridview header cells
           for (int i = 0; i < gvEmpdetails.HeaderRow.Cells.Count; i++)
               gvEmpdetails.HeaderRow.Cells[i].Style.Add("background-color", "#df5015");
           gvEmpdetails.RenderControl(htw);
           Session["ResponseData"] = sw.ToString();
           iframe1.Attributes.Add("src", "Download.aspx");

When user the Open button in the popup excel is open it is working correctly.

Simliarily When user Click Save button in the popup the user select the path where excel is to be there and save the excel file.

for that saving the excel file what code i have to do.

What I have tried:

i have gridview and i am exproting that gridview data to excel.

Before downloading i show popup as  Open Save Cancel.

When user click the Open the excel is file open.

The excel open code as follows

<pre lang="C#">System.Threading.Thread.Sleep(3000);
           System.IO.StringWriter sw = new System.IO.StringWriter();
           HtmlTextWriter htw = new HtmlTextWriter(sw);
           gvEmpdetails.AllowPaging = false;
           BindGrid();
           //Applying stlye to gridview header cells
           for (int i = 0; i < gvEmpdetails.HeaderRow.Cells.Count; i++)
               gvEmpdetails.HeaderRow.Cells[i].Style.Add("background-color", "#df5015");
           gvEmpdetails.RenderControl(htw);
           Session["ResponseData"] = sw.ToString();
           iframe1.Attributes.Add("src", "Download.aspx");

When user the Open button in the popup excel is open it is working correctly.

Simliarily When user Click Save button in the popup the user select the path where excel is to be there and save the excel file.

for that saving the excel file what code i have to do.