The v15.1 installer is several gigabytes. Use a download manager if your connection is unstable to prevent corrupted ISO files.
If you are still running an older version (e.g., V14, V15.0, or V15.0 SP1), you are missing critical updates. Here is why the Control Expert 151 is mandatory: control expert 151 download updated
If you choose to stay on version 15.1, ensure you apply (or later). This specific update fixes a critical issue where PLCs could crash (3 RED LED status) when restoring data from a file while the controller is in RUN mode. Upgrading Control Expert 15.0 SP1 to version 15.1 The v15