If you’re having trouble reinstalling Dropbox on your Mac, follow these steps:

  1. Quit Dropbox if it’s open.
  2. Open the App Store on your Mac and search for “Dropbox.” Click the app’s icon to install it.
  3. Open Dropbox and sign in if you don’t already have an account. If you do have an account, click the “Your Files” tab at the top of the window and select all of your files to upload them to your new Dropbox account. (If you only want to import some of your files, select just those that you want to keep in Dropbox.)
  4. Click the “Reinstall” button at the bottom of the window and follow the instructions onscreen. ..