Datagridview row filling using value in first cell

var ComboBoxValue = ((Combobox)GridView1.Row.FindControl("Combobox1")).selectedValue();

If your comboBox is populated for example, with the column 'Name' then you have to write the required ADO.Net in order to gain access into the database and make the appropriate changes.

using System.Data;
using System.Data.sqlClient;

DataTable table = new DataTable();

using(sqlConnection connection = new sqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["preffered Connection string"].ConnectionString.toString()))
  {
    connection.Open();
    using(sqlCommand cmd = new sqlCommand(String.Format(@"Insert into tbl_Clients("+ "LASTNAME" + "," + "PHONE" + "," + "CELLPHONE" + ")         Values('{0}','{1}','{2}','{3}') Where Name ={4}",value1,value2,value3,value4,ComboBoxValue.toString()),connection)){
         using(sqlAdapter adapter = new sqlAdapter(cmd) ){
             adapter.fill(table);
         }    
        
      }
  }
 GridView1.Datasource = table;
 GridView1.DataBind();

If you would like to populate a single Row then you have to be specific about it and specify the required Row and cell in order to do so.

OnSelectedIndexChanged(object Sender,EventArgs e){
   var DataRow = e.Row
}

This is an event of GridView of which the EventArgs variable might give you the specified Row that the changes are taking place.

Hope this helps!!
Best Regards