how can I change colour of new octavia dash and centre from green to blue please

You are going to need to either source all the parts from a VW (Golf?)

Or get useful with a soldering iron and change all the LED's, plus you will need quite a bit of patience!

EddyH did this exact thing on his 4x4 Octavia a few years back, this link tells you all about it (the pics have long gone, but it gives you an idea on what is involved).