Trek Valencia+ HELP ME PLEASE

Hello, I am considering buying my friend's Trek Valencia=+ ...there is only one problem. He let the bike sit idle for around 7-8 months. Now the battery only gives me about 8 miles of life, even conserving it as much as possible. I need to A) find out EXACTLY where and how much a replacement battery costs, B) UPGRADE the battery as described above. Can you tell me where I am to purchase such a battery online, or do I have to go through a BionX dealer? I am not an electronics wizard, however the LiFePO4 batteries look like a way to go, but wil I need to change everything to add this kind of battery? I am very confused, and perhaps I will also post this as a new thread... Thanks in advance for any help, Petey