Use the adjective changing to describe something that doesn't stay the same, but continually alters or changes with time.
When you talk about the changing seasons, you're referring to the dynamic movement from fall to winter and winter to spring — along with all the differences between one season and the next. You could also describe the changing population of your city, in flux and undergoing constant shifts and variations. Changing comes from the verb change, with its Old French root changier, "to alter, exchange, or switch," and ultimately from the Latin cambire, "to exchange or barter."