wp-user-meta.php

<?php

function render_user_profile_fields( WP_User $user ) {
	$twitter = get_user_meta( $user->ID, 'twitter', true );

	?>
	<h3>Custom user info</h3>
	<table class="form-table">
		<tr>
			<th>
				<label for="office">Twitter username</label>
			</th>
			<td>
				<input type="text" value="<?php echo esc_attr( $twitter ); ?>" name="twitter" />
			</td>
		</tr>
	</table>
	<?php
}

add_action( 'show_user_profile', 'render_user_profile_fields' );
add_action( 'edit_user_profile', 'render_user_profile_fields' );

function update_user_profile_fields( $user_id ) {
	if ( ! current_user_can( 'edit_user', $user_id ) ) {
		return;
	}

	update_user_meta( $user_id, 'twitter', sanitize_text_field( $_POST['twitter'] ) );
}

add_action( 'personal_options_update', 'update_user_profile_fields' );
add_action( 'edit_user_profile_update', 'update_user_profile_fields' );