Use [`update_post_meta()`](https://codex.wordpress.org/Function_Reference/update_post_meta), because if the meta key already exists [`add_post_meta()`](https://codex.wordpress.org/Function_Reference/add_post_meta) will do nothing if `$unique` is `true` and otherwise won't update neither, while `update_post_meta()`, well, it will update the value of a existing field or create it, if it doesn't exist yet.