On the "Files" tab, click the "Upload file(s)" button.
Select your file(s), you want to upload from your computer. You can upload multiple files to save time (if required).
Files will be uploaded into the folder of which is selected (eg "My files") in the right hand side (highlighted with a yellow bar).
Also if you upload file(s) with the same name into the same folder, you will be asked if you want to overwrite the files on the server.