You can't, only if you have done so in the past when 6.1.3 was still signed by Apple's servers AND used a tool like iFaith to save the so called SHSH blobs that are unique to your device and change when you change the iOS version.
I suggest you do it now and save your blobs (it's a single file) in multiple locations so you never lose it. Then you should be able to downgrade to 6.0.1 at any given time (currently you need iTunes 11.0.5 to do that).
Like was mentioned earlier, if you love iOS 6 then backup those blobs. iTunes will not allow you to go to any firmware but the current for your device.