how do I treat the leaves on this plant? I think they are sunburned, from reading this.
Only new leaves growing out are brown.
Certified GKH Gardening Expert
If these are outside, then possibly. If they are in window light, then you won't, usually, have to worry about sunbleaching.
This can also be caused by overwatering, or not letting the soil dry out just a touch before watering again. The most common cause of what you are seeing is, exactly, this.
If moving to a less heavily lit spot doesn't help, you will want to look to a fungicide.
This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm