You shouldn't need to uninstall the previous version btw, just run the installer.
What OS are you using?
Did you download it from the official calibre web page?
Is the downloaded file named calibre-0.8.47.msi (if you are a Windows user)?
What does calibre say down the bottom left - no trace of an asterisk there?
Edit - got distracted and others more knowledgable have posted in the meantime
