我现在不想谈那件事。

I don't want to talk about it now.

I'd prefer not to talk about it. (我不想谈论那件事。)

那事儿以后再说吧。

Let's talk about it later.

例句：

We're playing golf this Sunday, right? (我们这个星期天要去打高尔夫球，对吧?)

Let's talk about it later. (那事儿以后再说吧。)

另外我们说说……

To change the subject...

例句：

I can't believe it. (真让人难以相信。)

To change the subject... (另外我们谈点儿别的吧。)