Panopto has an excellent introduction to basic editing on their website.
Then here are instructions on how to add/remove slides from a presentation.
Suggestion — If you only need to add a few slides to an existing presentation, make sure the PowerPoint file you upload only contains those few slides. It will make the editing process a little easier.
For those who are a bit more adventurous, you can insert one presentation into another. This is great if you need to add a section with both audio and slides to an existing presentation.
Please contact us at firstname.lastname@example.org if you have further questions.