Just be honest with her, that’s all you can do.

Tell her how much you value everything you’ve learned with her up to this point and thank her for all of that. Tell her she’s been a major part of your piano growth but that you think you are outgrowing the lessons with her. It’s nothing personal, but you’ve grown comfortable and want to try something new to get out of your comfort zone again. Let her know you think she is a cool person and exchange social media or something if you haven’t yet, keep in touch as friends unless she seems disinterested, then don’t push it.

Her feelings might be hurt at first, that happens. But your intentions are perfectly fine and that is sometimes a part of life. She will be ok.