How do I turn down a project I'm not interested in anymore?
A polite and concise email to the postdoc will suffice.

As an undergraduate in my early years, I think I'm still figuring out my interests

A postdoc researcher will certainly understand this sentiment. There is no need to go into details about you not finding their work interesting, but rather focus on changes in your interests.

Since I haven't been offered an official position at the lab yet, I think this is still a good time to make this call.

The postdoc will be glad that you did not waste their time and they will appreciate knowing about your decision to not join their lab ahead of time.