I'm so sorry but your country Nigeria is not a PayPal supported country.
WordPress.com uses PayPal as their payment processor, and as such, can only accept payment from countries they support: https://www.paypal.com/worldwide/
PayPal have chosen to block payment from your country, so you will not be able to purchase an upgrade here unless you have a source of payment outside of your country.
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:
WordPress.com also accepts Bitcoin for payment of certain upgrades ( bitcoin cannot be used for domain and premium theme purchases) - you can learn more here: http://en.blog.wordpress.com/2012/11/15/pay-another-way-bitcoin/