1. Install Upgrade Status[https://www.drupal.org/project/upgrade_status] module to check issues contrib and custom modules compatibility.
2. Remove Upgrade Status when all checks are green and ready to go. Uninstall Upgrade status and remove it from Composer.
composer remove drupal/upgrade_status3. Temporarily add write access to protected files and directories
chmod 777 sites/default
chmod 666 sites/default/*settings.php
chmod 666 sites/default/*services.yml4. Upgrade Drupal Core and modules
composer require 'drupal/core-recommended:^10' 'drupal/core-composer-scaffold:^10' 'drupal/core-project-message:^10' --no-update5. If Drush is present, upgrade to 12
composer require 'drush/drush:^12' --no-update6. If core-dev is present, this might be required
composer require 'drupal/core-dev:^10' --dev --no-update7. Do a dry run
composer update --dry-run8. If no errors, upgrade
composer update9. Verify again if no errors on update in step before with install
composer install10. Update Database and Export config
drush updatedb -y && drush cex -y11 Restore read only acces
chmod 755 sites/default
chmod 644 sites/default/*settings.php
chmod 644 sites/default/*services.ymlLinks
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https://www.drupal.org/docs/upgrading-drupal/upgrading-from-drupal-8-or-...