A person from a visa exempt country can stay up to 6 months legally in Canada under the visitor status. Since you have no temporary visa issued, how can you apply for an extension??
Simply exit the country and re-enter in order for you to get a new stamp in your passport. This stamp automatically gives you another 6 months from the day you re-entered the country.
However: it is up to the discretion of the officer at the point of entry to grant you a stamp or not. If he is not satisfied, he may grant you entry but not stamp your passport. In that case, the older stamp of the initial entry to Canada is valid until expiration. Even if the officer refuses to give you a new stamp, simply ask for one, sometimes it works.