To disable WordPress Appearance Theme Editor and Plugin File Editor, you can use:

• iThemes Security Free
• or edit WP-Config.php

Method 1. Using iThemes Security Plugin

Step 1. Install and activate iThemes Security (formerly Better WP Security)

Step 2. Security > Settings

Step 3. WordPress Tweaks > Configure Settings

Step 4. Scroll down, tick on Disable File Editor > Save Settings

Step 5. Refresh website

This plugin not only disables Editor in Appearance but also File Editor in Plugin

Method 2. Editing Wp-config.php

Step 1. Go to Hosting > File Manager > Find you website which you want to disable File Editor

Step 2. Right Click on wp-config.php > Edit > Edit

Insert this code into wp-config.php, under <?php

define( 'DISALLOW_FILE_EDIT', true );

Done!