Add foreign key to table after migration has been run in laravel : I have the following migration in my laravel migrations folder that i have already run:

    <?php
    
    use Illuminate\Database\Schema\Blueprint;
    use Illuminate\Database\Migrations\Migration;
    
    class CreateAdminTable extends Migration
    {
        /**
         * Run the migrations.
         *
         * @return void
         */
        public function up() {
            Schema::create('Admin' , function($table){
                $table->increments('id');
                $table->mediumText('title');
                $table->text('blog_content');
                $table->char('tag' , 15);
            });
        }
        /**
         * Reverse the migrations.
         *
         * @return void
         */
        public function down() {
            Schema::drop('Admin');
        }
    }

The above migration is for my `admin` table, what i would really like to do is add a foreign key in my `admin` table that is associated with my `tags` table. something like:

     $table->foreign('tag')->references('tag')->on('tags'); 

How do i do this now that i have already run my migration ??

Thank you.