Get the process id which is running on the specific port
sudo lsof -i :8081
Once you get the process id then you kill
kill -9 [pid]
Useful drush commands while developing your hook_update_N
Checking the current schema version of a module
drush php-eval "echo drupal_get_installed_schema_version('my_module');"
if you want to set version then use the below
drush php-eval "echo drupal_set_installed_schema_version('my_module', '8000');"