Use below command to modify the existing table
php artisan make:migration add_shipped_via_and_terms_colums_to_purchase_orders_table --table=purchase_orders
use --create for creating the new table and --table for modifying the existing table.
Now a new migration file will be created. Inside the up() function in this file add these line
Schema::table('purchase_orders', function(Blueprint $table){
$table->string('shipped_via');
$table->string('terms');
});
And then run php artisan migrate