Heraclitus believed in empiricism which is the view on how knowledge is earned through your sense of touch, smell, taste, etc. The substance that Heraclitus uses to represent change is fire because fire is always changing. He looked at the world different when he was figuring out his fundamental substance but we now know what the fundamental aspect is change. The change resulted from a battle with opposing forces like old vs. young. Anything that changes can never be the same again. People cannot step in the same river more than once because the river is always changing. His problem was that he said that everything and everyone is always changing.
This expression can be related to anything that can change, which is everything. Only change is real while identity and stability are illusions. He did not consider the change random; instead he saw a cosmic order as the cause of all the changes. He named this process logos which is Greek for word. The logos is based off the knowledge he had.