How does electromagnetic energy change?

Electromagnetic energy is determined by wavelength and frequency. These are dependent on each other, so anything that changes one also changes the other.

The fundamental formula for this is wavelength x frequency = speed of light.

Since the speed of light is a constant then the other 2 variables are inversely proportional (when one increases the other decreases).

What cause the changes is the interaction with something. For example, when visible light from the sun hits the Earth it warms up the Earth and the Earth emits heat. Visible light has a wavelength of approx. half a micron. Heat has a wavelength of about 10 microns. So the electromagnetic energy will have increased it's wavelength and decreased it's frequency.

Electromagnetic energy can be transformed into different types of energy, like heat or mechanical energy, by a variety of processes, including absorption, emission, reflection, and transmission.