You cannot upgrade the battery. In theory they could sell extended batteries for it, but this is unlikely to happen because (IMO) Nexus sales are not high enough to support it.
I use mine for web browsing, music, and reading books mostly. I can usually go a full 12 hour day from 100 to zero. I typically charge while at work, but not anywhere else...so my charge will last all night and through the next morning, usually with 20-40% to spare. I do not dim the display, but I do turn the phone off when I'm not using it. Battery life in not awful, but is not great. This battery withers under gaming too...so if you are a phone gamer, I'd recommend getting one with an extended battery. Also, the hotter it is, the faster it loses it's charge. If you live in a warm climate, battery life will be worse.
The battery is very good at holding a charge though. If I go to bed with 63% remaining, I will wake up with 63% remaining. When the display is off, it is using virtually no power.