You have to change you directory to the flutter installation folder.
Alternatively you can just type the following command:
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flutter upgrade --force
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I want to upgrade most latest version. So I used below commands:
flutter channel dev
Flutter has 4 project branches / channels ie stable, master, dev, beta. First
when we install it. flutter install stable channel. If you want to switch :
flutter channel chanel_name
Then donwload that :
flutter upgrade