How do I upgrade the PHP version used?
The latest version of Acquia Dev Desktop (released on May 21, 2020) is compatible with Drupal 9: It added support for MySQL 5.7 and Drush 10; PHP has been updated to versions 7.2.28 and 7.3.15.
Since Drupal 9 requires PHP 7.3 or higher, you need to select version 7.3.15 for Drupal 9, or you will get the error message you saw.
References

The release notes for Drupal 9

The download page for Acquia Dev Desktop