How do I upgrade the PHP version used? I've been away from Drupal a few years. I now want to develop a site. I see it's up to version 9. However, when I tried to install Drupal 9 in DevDesktop to get a feel for it, I got the message PHP version too old. I'm selecting the highest PHP version, 7.1.12, and still get this message. I know D9 likes PHP 7.5 or something, but how do I get that?
Should I simply upgrade PHP locally, independent of DevDesktop? Or is DevDesktop now old school, replaced by something better?
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