It is easy to change the default ‘set featured image’ text in wordpress admin side.
Please add the following code in the function.php file in the theme folder

function custom_admin_post_thumbnail_html( $content ) {
    return $content = str_replace( __( 'Set featured image' ), __( 'Set default image' ), $content);
}
add_filter( 'admin_post_thumbnail_html', 'custom_admin_post_thumbnail_html' );

Now ‘Set featured image’ text will be changed to ‘Set default image’.

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