I want to modify the corev15.css file for my team site inside SharePoint 2013, but i am not sure where is this file located inside my windows 2008 r2 server ? can anyone advice ?
You should not modify SharePoint's out of the box style sheet files, they are provided by SharePoint, and used across all sites, if you need to override these styles, you need to create your own style sheet file (css file), place it inside SharePoint, and reference this css file, this way you can overwrite the styles that come by default in SharePoint
As mentioned in the answers on this question about the location of the _layouts folder (and it's where corev15.css is located), you can access this folder in:
c:\program files\common files\microsoft shared\web server extensions\
Then in the web server extensions folder search for "LAYOUTS", now depending on the path where your css file is located (for example: .../_layouts/15/1036/styles/corev15.css) you open its _layouts folder, this is it for our example:
After you open the folder, you follow again what's in corev15.css link and you find your css file (like in this picture):