How do I change a factions starting position

[Robertclive]

I want to change Spains position to Cuba,and England's to North America.Will I have to do religion and culture editing too.

[Hossi]

That's pretty easy. Just edit the descr_strat.txt. Theres a chapter for every faction, and an entry for every region. Just cut and paste the region entry's to the faction you want to have them.
Oh, and don't forget about the genearls/armys. You'll want to switch them to, otherwhile you'll and up with an rebel army in your english city.
And finaly add an
restrict_strat_radar false
to your camaign_script, to have the america-map from the beginnig.
Religion and culture shouldn't be a big problem, just parse throuh the city entrys before you change the owner, to make sure there's nothing in there that the target faction can't have, like a 'temple_pagan high_temple'.

[Eothese]

Will I have to do religion and culture editing too.

Not unless you want to, but as these provinces are both at 100% pagan to start there may be an issue with religious unrest. As for culture there is no need to make any changes.
Just remember that any changes made to any files in the base folder will only show up if you delete map.rwm, even text files.
Don't worry though, the file regenerates itself when you launch a new campaign.