First sign in to the Dropbox website and install Dropbox application. Then:
- Click the Upload button at the top of the window.
- A window will appear. Click the Choose File button and then select the file on your computer that you'd like to add to your Dropbox.
- Choose as many files as you like, and then click the Start Upload button.
For more information about Dropbox check their Help Center at https://www.dropbox.com/en/help/90
