Pretty simple, would they tell their pasts selves something of importance? Kill an old enemy? Save Lincoln?
Izaiah has already managed to save his mother (Abigail Knight) by time travel, though they have reason to think that may have been a unique event (i.e. changing the past is usually more problematic).
Other than that, he'd tell his past self to not kill Aton, but find another way to stop him and prevent his future escape.