Type in your terminal:

flutter upgrade

This will update Flutter to the latest version in the current channel. Most likely you have it set in stable.

flutter channel
# Flutter channels:
#   beta
#   dev
#   master
# * stable

Do you want to live in the cutting edge? Switching channels is easy:

flutter channel dev
# Switching to flutter channel 'dev'...
# ...

And run upgrade again:

flutter upgrade