Percent change is arithmetic. Determine how much a measure has changed (or perhaps how much it’s expected to change) and divide it by the original level and that’s percent change. Simple enough.
Where it gets complicated is how we talk about this arithmetic. In economics, we don’t talk about one percent change, we talk about many—and the difference is in which data points we choose to compare. For instance, these are a couple of ways you could interpret the US’s second quarter GDP.