You have to talk to Nook and select the option about your house and then he will give you an option to customize it. You unlock more customization options as you upgrade your house. You first get the option to do it after upgrading to the left room.
so when your house is paid off, you can go talk to Nook in the Resident Services building and he will allow you to change your house exterior for free once per day. you might also talk to him to change it before you pay it off but it's been a while so I can't really remember