Windows that are NOT replacement windows need to be replaced from the exterior of the home. There is a nailing flange on the window that needs to be nailed to the exterior sheathing. This nailing flange is then covered with flashing and your exterior window trim which your siding butts into. Although, if the windows being installed are considered replacement windows, then the installation process is slightly different. Replacement windows are installed inside of the old window frame. Going with replacement windows is typically a more cost-effective route although it may not be as aesthetically pleasing as getting non-replacement windows.