Classic ASP.NET Web Application is a front-UI (aspx) and backend code-behind page (in c# or VB). If you just change on the html or style, it is find to update it in the aspx page. But if you are going to change the code logic, you will have to do it in the code-behind page. And you will also need to compile it after modified. And you will also need to re-deploy the files (specially the update aspx and files in bin directory).
Do you know which version of the .net that your asp.net web application is?