The battery is probably locked in place by one or two sliding clips so remove the old one and use the printed information to find and buy a replacement. Beware of cheap batteries on e-Bay - some of them self-combust. Pay good money for a decent battery then fit it and make sure it clicks into place. Run the laptop until the battery actually turns it off then charge it fully and do that half a dozen times to train the battery.
Just make sure you save all your work when the battery starts to run down below 10%. Windows will probably shut down before the battery is flat out. Traning the battery that way wlil make it work better and last longer.