Which part of the plan needs changing?
Sometimes the answer is obvious. A whole wave goes past one tower, and it probably needs help. Other times you buy an upgrade and can't quite tell whether it was worth the money. Watch the next wave before spending everything that's left.
The Bloons TD series puts the action on a path. Age of War is a battle between bases, with units moving across the screen. Ages of Conflict takes a much wider view of a map. Choose according to what you feel like managing, rather than expecting the same controls in each.
Keep something that worked
On the next try, leave the useful parts alone. Move the tower that had a bad position or spend differently on the units that kept losing. Changing one thing makes the result easier to understand.
It isn't necessary to win the first attempt. Finding a way past the bit that stopped you last time is a reasonable place to begin.