How to Re-install an App that Shows up in the Appstore as 'Update' Instead of 'Buy App'

This fix is NOT supported by Apple's Official Support Policies

1. Open Terminal
   

2. Paste these codes in one line at a time:

   rm -r ~/Library/Caches/com.apple.appstore
   rm -r ~/Library/Caches/com.apple.storeagent
   rm ~/Library/Preferences/com.apple.appstore.plist
   rm ~/Library/Preferences/com.apple.storeagent.plist
   rm ~/Library/Cookies/com.apple.appstore.plist
   

3. Navigate to: Launchpad > System Preferences > Spotlight > Privacy (tab)

4. Click '+' in the bottom left
5. Select 'Macintosh HD' (click ok when prompted)
6. Leave it a few minutes and then click '-' to remove it
7. Type something in Spotlight and wait for it to begin 'Re-Indexing'
8. Once Spotlight has finished reboot
9. Install the App out of App Store (this should work now)
10. Once installed open 'Applications' locate app
11. Right click and select 'Show Package Contents'
12. Remove the file named 'embedded.mobileprovision'
13. Remove the folder called '_MASReceipt'
14. Reboot

When the Mac App Store launches it scans ALL apps regardless of if it was installed via MAS, it picks up the apps that were installed via MAS and then provides any updates, once the update has been triggered it gets embedded into those files (via an encrypted string) and remains there until cleared or updated.

This method removes the files that contain parts of the update string, when the app store cannot find the string it clears the update status and you should be good to uninstall and re-install via app store (I've just done this successfully) and/or update to a newer version.

This method was advised to me by an Apple staff member off duty and therefore does not actually represent Apple's Official Support Policies.

Go to updates in App Store. Click store in menu bar. Click reload page.

This worked for me.