Login to Google Classroom, navigate to "Classwork" on top of the screen.
Find the TP you want to upload material for, Click on it.
On the right side of the screen, select "Add or Create"
If the file you intend to upload is on your computer select "File" in the new window find your file and select it.
Click on "Upload"
But, if you want to add anything from Google Drive (used for the files you have created using Google Docs and Slides) select the file(s) and click on Add.