No, you can not extend your stay, unless it is some sort of emergency like hospitalization, canceled or delayed flight for more than 24 hours, etc help.cbp.gov
ESTA is usually granted for 2 years or until your passport expires (whichever comes earlier), so depending on when yours started it is possible, that you may not have to apply again help.cbp.gov
However, one of the conditions for granting the entry is that your travel may not terminate in adjacent country (like Canada, Mexico or any of the Carribean islands) esta.cbp.dhs.gov
Also, note that having an ESTA does not automagically grant you entry - that decision made by the CBP official www.cbp.gov
Therefore, you would have to travel some place else then Canada in order not to violate the rules. And you better be prepared to give a good answer to CBP official asking you why you are back so soon.