If you have 8.1 installed already you will see this message while trying to install 8.1.1:
In order to successfully install 8.1.1 follow these steps:
1. Create backup copy of your project files and libraries. If you have modified the standard libraries ensure you make a backup copy.
2. Then, uninstall current version (8.1):
3. Download and install the latest release version (8.1.1).
If you modified any of existing libraries you will be warned during the installation process with the message informing that your changes will be lost:
In order to preserve your modifications please chose a different folder other than \Library to store the new libraries when installed.
Another method to restore your modified libraries is to keep the proposed install location for library files, complete the installation and follow 4th point.
4. In order restore your backed up modified libraries that you prepared in point 1 simply copy them into default Library location:
C:\Users\Public\Documents\DesignSpark PCB 8.1\Library