Drupal 9 is clean up version of drupal 8.
Upgrading from drupal 8 to 9 is very easy, if you want to upgrade please
follow below the step.
Step 1: Install Upgrade Status Module https://www.drupal.org/project/upgrade_status
goto your module folder and run below command via composer
$ composer install --dev
$ composer require drupal/upgrade_status
Module uploaded you can check under module list
After Enabling you can see the compatibility for your drupal 8 version with drupal 9
Your system must also meet the minimum system requirements for the Drupal version you are upgrading to, e.g. PHP 7.2+. See
Make sure to update all contributed modules and themes. Most projects will improve Drupal 9 compatibility as the release of Drupal 9 comes closer. Ensure they are all compatible with Drupal 9. They would also still be compatible with your Drupal 8.8+ site. Use Upgrade Status to check their compatibility. You should cooperate with module maintainers and offer help where possible to make sure updates happen. Check the module's project pages for Drupal 9 plans as most key modules provided that. Read Help contributed modules prepare for Drupal 9 for more tips.
If you have custom code and/or themes, you should also confirm they are not using any code deprecated in 8.8.x or earlier for removal in 9.0.x, prior to attempting to update your site to Drupal 9.
Finally once every other component is Drupal 9 compatible, update the core codebase to Drupal 9 and run update.php