The frequency increases.
Here's an easy way to think about this. The wavelength is the distance from crest to crest, or trough to trough -- equivalently, it's the distance the wave travels in one period. This means that the speed of the wave is the wavelength divided by the period, or v=λTv=λT . But the frequency is just the reciprocal of the period, so v=λfv=λf. Clearly, if v increases and the frequency stays the same, the wavelength must increase by an equivalent factor.
