I don't want to be in a dating relationship with my long distance partner anymore

I'm in a long-distance polyamorous relationship and I just want to be friends with them again... I don't know how to do this.

Either in an email or over phone/skype, depending on how you two best communicate, you just gotta tell them. Explain what you liked about the friendship you two had before changing the relationship into a sexual-romantic one. Explain what’s not working about the current relationship terms. Ask if you two can dial back the sexual-romantic aspects while maintaining the other emotional and social connections. They may or may not be willing/able to do that, and this might require a temporary or indefinite break from any kind of connection, and that’s their right. But you just have to tell them how you’re feeling and what you want.