Calm down.

冷静一点。

Keep cool.

保持冷静。

Don't get so excited about it.

别因为这事儿太激动。

Don't go into hysterics.

不要歇斯底里。

Don't lose sleep over it.

别因为这事儿睡不着觉。

Don't be so sensitive.

别太敏感。

Don't fly off the handle.

不要这么冲动。

Don't lose your cool.

要沉住气。

Be cool. Everything will be all right.

冷静一点,一切都会没问题的。

Keep calm when problems crop up.

遇事冷静。