"I seen" is not a thing. It never has been and it never will be. Okay, maybe it has been. And maybe it still is. I hear it all the time—sometimes friends, family, and coworkers have said it—but it's wrong.
If you see something in the past, you saw it.
When you see something at a non-specific time in the past, you have seen it.
Saw is the past tense of see. That's all there is to it.
It's reasonable to mess up using the words have and seen together. It can get kind of confusing.
For example, you might think
I have seen that movie yesterday
is correct. It's not. You should be saying:
I saw that movie yesterday.
Like I said, you can only use have seen for an undefined time. As soon as you specify a date, you have to use saw.
I have seen that movie.