Switching countries in Billing info

Hi.

I originally bought my MacBook Pro while in Japan. My Apple account registered with this Pc is linked to Apple Japan.

I have since moved back to Kenya. I tried to update my OS, but it required I update my billing information before I can do that. I would therefore need to change my billing information to the Kenyan one. I have tried to do so, but I can't seem to make this change it.

1. How can I shift my information from my Apple Japan account to Apple Kenya Account?

2. How can I change my billing information from Japan to Kenya?