How do you upgrade Nook 'n' Go into NookWay in Animal Crossing City Folk

Solution 1:

You have to wait at least 15 days after you have renovated to the Nook and Go, and then you need to have spent a total of 80,000 Bells at Nook's store. When both of those conditions met, you will be able to unlock the NookWay upgrade.

Solution 2:

To upgrade to Nookway in Animal Crossing: City Folk, the player must buy/sell a total of 80,000 bells and the store must be Nook 'N' go for at least fifteen days. To upgrade to Nookway in the later games, the player must only buy/sell 65,000 bells worth of stock at Nook 'N' Go.

Source: http://animalcrossing.wikia.com/wiki/Nook_%27N%27_Go