To replace front brake cable,
1. detach brake cable from front wheel's brake band.
2. remove the front bottom tube cover and the front wheel.
3.remove the top cap from front top tube. (*there are 2 caps, so just the 1st one for now.)
4.gently pull the cable (togather with the outer tube) from the front of the handle bar till bare inner cable appear.
5.remove the second cap from the front top tube. pulling the cap upwards, then the cable joint part comes out with it.
6.this is the core of the front brake.
7.the stopper on the end of upper cable might be hard to remove. I had to use two pliers to avoid damaging the screw head.
the rest should be easy, but remember the spring for the tube locking button works as a cable guide, so make sure the cable go through the centre of the spring when you put them back.
sorry, i have to rush off now, i hope this is enough to help you.