There are several ways to create an internal cheat for a desktop game that uses Adobe AIR and ActionScript 3, but they all involve some level of reverse engineering and knowledge of the game's code and architecture.
One way to create an internal cheat is to use a tool like Cheat Engine or ArtMoney to search for and modify the game's memory values at runtime. These tools allow you to search for specific values in the game's memory and then change them to give you an advantage, such as increasing your health or giving you more money.
Another way to create an internal cheat is to use a tool like a DLL injector to inject a custom DLL into the game's process at runtime. This DLL can then contain code that modifies the game's behavior in some way, such as removing a cooldown timer or increasing the player's speed.
It's important to note that many game developers consider cheating to be a violation of their terms of service and may take action against players who are found to be cheating. Additionally, it is illegal in some places to reverse engineer, modify or distribute copyrighted material without permission from the copyright owner, you should be careful when doing this.