Php version mismatch while upgrading laravel project

I am trying to upgrade laravel 5.4 to 5.6.

I am getting following error:

Problem 1
    - This package requires php >=7.1.3 but your PHP version (5.6.37; Package overridden via config.platform
(actual: 7.3.6)) does not satisfy that requirement.
  Problem 2
    - laravel/framework v5.6.9 requires php ^7.1.3 -> your PHP version (7.3.6) overridden by "config.platform
.php" version (5.6.37) does not satisfy that requirement.

How can I fix this?

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