How do you upgrade tom nooks store on animal crossing wild world

You upgrade Nook's store by purchasing and/or paying your mortgage.
There are four stores in all:

• Nook's Cranny
• Nook n' Go
• Nook Way
• Nookington's

Each store has a certain amount of money that you need to upgrade it.

• Nook's Cranny to Nook n' Go = spend $25,000
• Nook n' Go to Nook Way = spend $90,000
• Nook Way to Nookington's = spend $240,000 and have a friend shop at the store

So there's is the amount of money you need to spend in order to upgrade each store. For me it was a lot easier just paying off my mortgages one at a time. Usually this method works.