If the file has already been shared you will see Shared with these people followed by a list.
Click on the Share button.
Under Share with others enter a person's name or username, then click on Search for people.
Click in the box next to the person with whom you wish to share the file, then click on Confirm.
When you have done this you will see that the file has been shared with that person, and you can also specify Write permission. Tick this if you want the person to be able to edit and upload a newer version of the file.