Nigerian student in Japan wants to travel to China to renew passport

I am a Nigerian student currently doing a Master's Degree in Japan. I will like to go to Beijing, China, to renew my passport at the Nigerian Embassy there as the Embassy of Nigeria in Japan cannot handle that at the moment.

Which type of visa do I need to apply? Can I apply through an agent or directly to China Consulate in Japan? How much will the application cost? Do I need any invitation letter in this case?

Thank you