How do I install new battery on Acer aspire laptop then let battery drain

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.