The frequency stays the same and the wavelength increases.
Explanation;
Sound waves are a type of mechanical wave which requires a material medium for transmission. The transmission of such waves occurs as a result of vibration of particles in the material medium. The faster the vibration of the particles the higher the speed of the sound waves.
When a sound enters a new medium where the sound travels faster, it means the vibration of particles is high, and the wavelength of the waves increases which means the speed of the wave has increased. The frequency on the other hand will remain constant.
