How to resolve the issue with updating Drupal 8.2.x to 8.3+ with Drush
1.) SSH into your server, if you're updating on a remote webhost, and proceed to the root of your Drupal directory. This will normally be your public_html directory on a webhost.
2.) Use the following command to remove the vendor directory:
rm -rf vendor
3.) Then use the following command to remove the composer.lock file:
4.) Then run the following command to update Drupal:
composer update drupal/core --with-dependencies
Easy as that! You should be on 8.3+ now.