Answering the question: To upgrade to 180mm from 160 you need a Post-Post 180mm adapter and a new disc. The adapters are widely available as 180 is probably the most common sized front disc on MTB's.
I think the question that should have been asked is "My front disc is not working as well as I think it should, how can I make it better". As many say, its not size that counts.... (I could be wrong, maybe you just want to do the bicycle equivalent of stuffing a sock down the front of your pants, in which case, go for it, no need to read on)
A bigger disc is a cheap way to make cheap crap brakes work better, but they are still cheap brakes, so manufacturers have sold us the idea that you need big disks because its cheaper to make big discs than good brakes. They then go on to sell the idea that by spending more money, for the same weight you get better brakes.
If you have cheap crap brakes a better (although more expensive) option than a bigger disk is upgrade to better brakes. If you have good brakes then something is wrong if they are not performing well and you should address the performance problem (poor bedding in, contaminated pads etc).