l wanna get this car but l dont like the interior.. is there any way to change it? what would the extra cost be?

it would'nt be worth your while,unless your friends with those pimp your ride guys.just get another model or make,
If you really want it,you could get new seat covers and floor mats,cost about£30-£50 from Argos..
why not look at a Citreon C2 OR 4, They are quite sporty looking ,but economical at the same time

I'd get something else. It is not really feasible to change a car is interior apart from seatcovers, and since it is the inside of a car you see all the time you're in it, you'd be better off getting one you like.
Also a Peugeot 207 is not a particularly great car.

Look for different trim levels of 207's. There are quite a few, such as S, SE, GT, Urban. They will all have different seat upholstery and interior details. The top of the range GT has very nice part leather seats. The interior you do not like wo not be in ALL 207s, so have a look around before you give up, as 207s are fantastic cars.