How to update custom user meta field in wp?

by

You are using the wrong meta key:

update_user_meta( $user_id, 'Save Posts', $_POST['save_post_external'] );
update_user_meta( $user_id, 'Save Posts', $_POST['save_post_internal'] );

And that should be:

update_user_meta( $user_id, 'save_post_external', $_POST['save_post_external'] );
update_user_meta( $user_id, 'save_post_internal', $_POST['save_post_internal'] );