In game, there are various articles that involved in life, trade, weapon, clothes and so on. You cannot collect or find all the articles you need only by yourself. So you have to exchange the relevant things with others.
First, choose the one you want to exchange with, and then click the exchange icon.
When he or she agrees to exchange with you, an exchanging window will appear. First, input the price he or she asks for, then tick off the confirm button. Make sure what he or she gives is what you want, and click confirm button.