Inventor Nastran 2021.1 uses a major upgrade installer. If you have Inventor Nastran 2020.0 or 2021.0 installed, you do not need to uninstall it to install 2021.1.
Inventor Nastran 2021.1 can be installed from Autodesk Desktop App, in which case the default install location is on the C: drive. Inventor Nastran 2021.1 can also be downloaded from manage.autodesk.com, in which case the default install location is on the C: drive, but you can choose to browse to a different location if desired.