I just want to make sure one thing ....  I want to upgrade my dnn from 4.6.2 to 4.8 .   The upgrade process will add info into web.config file or replace a new web.config file ??? I have a lot of modification to the web.config file and I hope I will not do the modification again.

To add to what Sebastian has already said - the upgrade doesn't add a new web.config for you. Typically, you'd see a release.config included with the upgrade. You should basically use this and copy over your changes from old web.config (most important to remember is the machinekey stuff).... I don't remember the specifics but I'm pretty sure 4.8.0 is different from 4.6.2 (IIS 7 stuff was added in 4.8 or 4.7)...

Sanjay