Run the installer. It does not interfere with your modern ROBLOX installation because it runs in a sandboxed environment. Ensure you have enough hard drive space (approx 2-4GB) as it downloads all the legacy assets.
Go to the official Bitl repository or the Novetus website (avoid third-party re-upload sites to prevent malware). As of the latest updates, Novetus is version 2.x or higher. novetus 2016 client
Open the Novetus launcher. You will see a drop-down menu labeled "Client Version." Scroll down until you see "Client 2016" or "Client 2016e" (The "e" stands for "extended" or a later revision of the year). Run the installer