Always undo the negative first and connect it last.. that way you'll avoid welding your spanner to the body.
This is based on doing the job on a 156, I dont think the 147 will be any different.
Remove the 2 philips screws that hold the positive terminal block to the battery and then lever up the plastic bit they go into, a bit like a wall plug.
Loosen the positive terminal clamp and lift the whole terminal assembly up, you might need to undo some cable clips to lift it up enough.
Loosen and remove the 13mm nut holding the clamp to the bottom of the battery on the left, be carefull not to drop the nut.
Refit in reverse, the new battery should come with a bit of clear grease on the treminals, if it doesnt then put a small bit of vasaline on them.