I would first copy the data from the dropbox drive to a new folder on your desktop. Then I would uninstall everything Dropbox from the computer. Verify the data on your desktop is matching everything you find on dropbox.com after logging into your account. From there I would Move the Desktop backup folder to wherever you want to house it permanently.
Now I would reinstall dropbox and select the folder you would like backed up to drop box , tap or click here to see how.
Choose which folders to store on your hard drive
Open your Dropbox desktop app preferences.
Click the Sync tab.
Under Selective Sync, click Choose folders (Mac) or Selective Sync… (Windows).
Check the folders you’d like saved on your hard drive. Uncheck the folders you’d like removed.