You upload from outside the droplet, not while inside of it.

You should have what you want to upload in your machine.

The easiest way to copy is with scp command. scp is a command to copy over ssh.

An example of usage would be scp /path/to/your/local/file root@XXX.XXX.XXX.XXX:/path/to/where/you/want/to/put/the/file

Change root if you are logging into your droplet with another user.