After downloading the appropriate file, click on Open Folder and note where the download ended up. Run RockSim, select File/Reload engine data, and note where RockSim is looking for the engine files. Then move the data file to the same folder.
Run the RockSim program and select "File/Reload engine data..." (the new file should show under "Available engine files").
Next, find the old Cesaroni (CTI) engine file under "Selected engine files" and remove it by clicking on the "<=== Remove" button. Then under "Available engine files", select the new file and add it by clicking on the "Add ===>" button. Click OK to return to the RockSim main page.