When you are in the library you want to upload and if you have the right to do it:

Option 1

Simply click on the “upload” button.

Depending of the site, you could have a choice to make.

Your file explorer (in windows) or the finder (Mac) will open. Select your file and confirm it

Option 2

Drag and drop your file directly in your browser.

If you are allowed to upload, a gray box will appear in the library. Otherwise, nothing will happen