I can think of three way you can do this
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Log in to the Admin Control Panel and go to:
Users -> All Users
Find your username and click on Edit.
Check “Disable the visual editor when writing”
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Add this code in the themes function.php file
add_filter( 'user_can_richedit' , '__return_false', 50 );
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Install a plugin like Disable Visual Editor WYSIWYG