Why is my coolant brown and overfilled?

I have a 2011 Subaru Impreza, and lately the engine overheat light has been turning on. I stopped driving it.

I took a look at the coolant reservoir today. It’s full almost right to the top and it’s thin and brown.

Is it possible that flushing it out and putting new coolant in will fix my overheating problem, or should I just take it straight to a shop?