Though the instructions at Microsoft website is really easy to follow to install Dotnet core 2.0, I realized that the web page does not have instructions that can be easily followed to remove Dotnet core preview 2.

https://www.microsoft.com/net/core#linuxubuntu

Run the following commands to removed Dotnet core 2.

sudo apt --installed list

sudo apt --installed list | grep "dotnet-sdk"

sudo apt-get remove dotnet-sdk-2.0.0-preview2-006497

dotnet --version