How can you upgrade Nookling Junction into T&T Mart in Animal Crossing: New Leaf? I don't know how to upgrade Nookling Junction.
According to the Animal Crossing wiki:
To upgrade to T&T Mart, the player must have spent 10,000 Bells towards their House Mortgage and 12,000 Bells on items at Nookling Junction. The player must also wait for 10 days after the Town's creation before the option to upgrade is made available.
So spend lots of money in the store, and start paying off your mortgage. That will upgrade the store.
According to animal crossing new leaf wikispaces
Ten days after your Town's creation, and after spending 12,000 bells or buying 15 items, and after upgrading your tent to a house, Timmy and Tommy will find it profitable to upgrade their shop to T&T Mart. This improved store is open for more hours and has space for one extra furniture piece, fortune cookie, and tool. It also adds brand new items such as medicine, a set of wallpaper, flooring, and a feature called the Catalog. The 'Catalog' allows you to order by mail any saleable item you have had in your pocket or purse (in your inventory).
This upgraded establishment finally looks like a real store, with a brand new convenience store look. It features an electric sliding door and large glass windows across the front of the store. The store's name and logo are now lit up on electric signs atop the building which stand out more during the nightly hours.
For Nookling Junction to upgrade into T&T Mart, the following prerequisites must first be met:
10,000 Bell home deposit paid to Tom Nook
12,000 Bells spent at Nookling Junction
10 days have passed since town creation