Hi,
To open .dwg file, you need AutoCad with AutoCad DWG Launcher. Since you have installed AutoCad you can double click on the .dwg file to be opened or open AutoCad > Click on the A button in the upper left corner on the screen > Open > Drawings > Browse to the .dwg file location and open it.
For changing the installation path, you should uninstall the AutoCad first, then reinstall it. During the installation process, select the location that you want the software to be installed.
Please let me know if you need further assistance.
Stay safe