lighten up

輕鬆愉快地打發時間

explanation :
If you see someone who is very serious you might advise them to 'lighten up'. This is advice to take a more optimistic outlook and not give in to dark moods. If someone takes a small matter very seriously, they may be advised to 'lighten up'.  

e.g. Andy went into a big lecture when I broke the rules, so I told him to lighten up.