Uploading any type of item can be done in one of two ways, depending on the quantity of documents you are uploading.
Uploading a Single File
Click the "Add document" link as shown below
Click Browse and locate the file you will like to upload.
Click OK and the file will upload.
Uploading Multiple Files
Click the Open with Explorer button; this can be found in the ribbon along the top.
You will be prompted for your username and password to authenticate, you will then be able to upload multiple items to the SharePoint site by dragging the files into the windows explorer window that appears.