Diagnosis
It's normal for these to run warm, even hot, especially the main router unit. They're powerful, fanless devices packed into a small case, and heat is how they dissipate energy. It's designed to operate at that temperature.
How to Fix It
Your job is to make sure it can shed that heat properly. 1) Check its location. It needs open space, not inside a cabinet or tucked behind a stack of books. 2) Make sure it's sitting on a hard, flat surface—not on a blanket or carpet which can act as insulation. 3) Ensure the vents on the bottom and the gaps around the middle aren't clogged with dust. A quick wipe with a dry cloth can help.
If That Didn't Work
If it's so hot you can't comfortably keep your hand on it for more than a few seconds, and performance is dropping (devices disconnecting), it might be defective. In that case, contact Google support.
Heads Up
The heat is a sign it's working hard. If you add more points to your mesh or have a lot of high-bandwidth devices, it'll work harder and get warmer. Just give it breathing room and it should be fine for the long haul.