I'm guessing its not built in so how do i edit my save to do that?
The capital moves on its own

... It will always be in the highest income province in your demesne, that has the same culture and religion as your ruler. If culture doesn't match any of the provinces, then highest income province of the same religion. If that doesn't exist either, then it just the highest income province that is capital.
So if you have two provinces with almost the same base income, and same culture and religion as the ruler, then you can see the capital bounce between fairly often, depending on which is leading in income at the time.
I don't know if you can edit it in the save game file, but if you can, it will be futile, as the capital moves back to the wealthiest province again when you start the game.
If you REALLY want to move capital to a specific province, then you need to give away any provinces that would be considered before it for capital, until the desired province is top of the list.