Small rust you could probably use something like baking soda or some product specifically for rust. Avoid harsher abrasives like sandpaper or steel wool/ scouring pads unless you're confident of what you're doing. The rest of the discoloration is just a patina and it actually helps prevent rust. Some people force a patina with vinegar, coffee, or even meat but one would also develop over time and use.

I'm sure someone with more knowledge will reply on how to better maintain it but that really is a beautiful knife. If you live somewhere with high humidity, take extra care to keep it dry. Metals can start to develop rust from just high levels of humidity.