database/migrations/2019_10_31_132937_add_foreign_key_to_tokens.php
<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class AddForeignKeyToTokens extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('tokens', function (Blueprint $table) {
            $table->integer('user_id')->unsigned()->change();
            $table->foreign('user_id')
                ->references('id')
                ->on('users')
                ->onDelete('cascade');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('tokens', function (Blueprint $table) {
            $table->dropForeign('tokens_user_id_foreign');
        });

        // Drop foreign and change column don't work in the same call to Schema::table
        Schema::table('tokens', function (Blueprint $table) {
            $table->integer('user_id')->change();
        });
    }
}
