How do you know the previous files are there? The installer should detect if you try to install it in a non-empty folder. If you are using a different path, the new installation should be isolated form the previous one, using different ports and files.
The way to manually remove a previous installation is to run the command below:
sudo pkill -f odoo
sudo rm -rf INSTALLDIR
Note that you should change INSTALLDIR to the folder specified during installation.