I'd like to return this. / I'd like to get a refundonthis.
我要退货.

I'd like to exchange this.
我想要换这个东西.

A: Excuse me.I'd like to exchange this blouse.
A: 对不起,我要换这件衬衫.

B: Whats the reason?
B: 什么原因?

A: Its the wrong size.
A: 尺寸不对.

B: Do you have the receipt?
B: 你有收据吗?

A: Yes,I have.
A: 是的,我有.

B: Go ahead and pick up the center size and bring it here,please.
B: 请去挑尺寸合适的来.

A: Ok, where is the fittingroom?
A: 好的,试衣间在哪?

B: It's behind you.
B: 就在你后面.