You must be set as "Presenter" to have access to the Upload Presentation button. You can see if you are presenter in the User list if there is an icon of a presentation board on the left of your name.
Click the "+" action button in the bottom left corner of the virtual classroom.
Click "Upload Presentation"
Drag and drop the file into the dotted box or click box to select from your device menu.
Select the file by clicking the empty circle.
Please note we recommend you use PDF files but other formats will work such as Microsoft Office.
See attached screenshot to help point you in the right direction.