To upload a file into the Files repository you must have the file saved on your computer first.  

Open up the File repository by clicking on the File icon as shown below: 

Once inside the Files section you can upload a document straight into this area by clicking on the 'Upload' icon as shown below: 

Alternatively you may with to add a file into a folder, in which case just open up the folder to upload your  new file into. 

Next click on 'Add Files' to select a file to upload:

This will take you into your computers documents - search your documents library to find the file to upload. 

Select the document with a double click and then you will see you can then click on the 'Upload' icon to add your file. 

The file will show as 100% when it has been uploaded. 

Click on Close to return to the main Files section.