Gigahertz are a measurement of how fast a processor operates. They can be "upgraded" in the sense that you can overclock a processor to get "more" of them, or you can install a better processor for the same effect.
A gigahertz is a measure of frequency. Giga means billion, hertz means frequency.
Therefore, a gigahertz is a frequency measurement of One Billion (1,000,000,000) hertz.