If you are talking about the HDD password, you can't. You'll have to change the Hard drive and load a new OS. If it's the Windows log on password, there are recovery programs that can help with that.

you can try accessing the acer recovery management tool by typing F8 key on boot up, a list will appear and you must choose "repair your computer" then another list of options and the last one "recovery management powered by acer" you will choose this one, and it'll take you to the recovery tool. from this point just follow the instructions. good luck, or some use the alt + F10