We use PayPal as our payment processor, and as such, we can only accept payment from countries they support: https://www.paypal.com/worldwide/

For whatever reason, they have chosen to block payment from Nigeria, so you will not be able to purchase an upgrade here unless you have a source of payment outside of Nigeria.
http://en.forums.wordpress.com/topic/billing-issue-because-of-my-location?replies=7#post-1052083

In addition to major credit cards Mastercard, VISA, American Express, Discover, and PayPal https://www.paypal.com/worldwide/ WordPress.com can accept echecks and some debit cards.

The debit card must have a VISA or Mastercard symbol on it, and should work just like a credit card when selecting the VISA or Mastercard option on the payment form, even though it's a debit card.

More about echecks here:
https://www.paypal.com/us/webapps/helpcenter/article/?solutionId=13054&topicID=11500008&m=TCI