Firstly, I recommend you update Windows Installer. If you're using Windows XP or below, you can update it using the package provided by Microsoft. If you're using Windows Vista or above, update it through Windows Update. After that, you need to know that you have to install Flight Simulator X Service Pack 1 before installing Flight Simulator X Service Pack 2. If it is installed on your computer, download Flight Simulator X Service Pack 2 from the official web page using the red Download button, save the file to your PC and install it following the on-screen instructions. I suggest you check the Readme page to check the minimum system requirements, the installation notes and all other related information.