Welcome to the Bunnings Workshop community. It's brilliant to have you join us, and thank you for sharing your question about your downpipe.
Your downpipe is held in place by what is called a blind rivet. In order to remove this blind rivet you'll need a drill and a drill bit that is slightly bigger than the hole you currently see in front of the rivet. You'll need to drill right through to dislodge the cap that is on the other side of the rivet. Once removed, you should be able to take the top part of the rivet off and disassemble the downpipe.
To put it back together you can either use a rivet with a rivet gun or a stainless steel screw. Using an ordinary screw would not be good as this would immediately rust and damage the downpipe.
If you need further assistance, please let us know.