What happens to speed of em wave as frequency increases?

Frequency = Speed of Light / Wavelength.

As you can see from this formula, when the wavelength increases the frequency decreases.

For example, we will say that the standard wavelength is 5m.
F = 3.0 x 108 / 5
= 60,000,000 hertz

Say if we increased the wavelength to 6m:
F = 3.0 x 108 / 6
= 50,000,000 hertz

As you can see, if we increase the wavelength, the frequency decreases.

If we decreased the wavelength to 4m:
F = 3.0 x 108 / 4
= 75,000,000 hertz

As you can see, if we decrease the wavelength, the frequency decreases.