lighten up

verb2

1. •
   To make (something) or to become less serious and more cheerful or casual; to relax.

   • I wish he'd lighten up a bit and realize that we were only joking.
   • The mood at work has been tense, so he's looking for ways to lighten it up.
2. •
   To lighten (something): to decrease its weight.

   • He can't get any more power out of it [his drag racer], so now he's looking for ways to lighten it up.