Have to tread carefully here. Best case is to tell her in person and don’t be rude to say that you don’t find her attractive enough, no matter what the Internet says, she will take it to heart and it will be a messy breakup.
Like it or not you will have to play that “It’s not you it’s me” crap. That is the reality. Else you can be down right honest as well. It all comes down to you.
If you go via polite route, then maybe say that you are not sure what you need from life right now(since you are 24 only) and want to explore on your own want you want in future.
Relationship is not only about physical traits but more about the overall person as well. So keep that in mind as well. Just my two cents🙂