Yes, you can apply for visitor visa extension within Canada. Please refer the below link:I will.be applying visa extension in October.
It's her visitors visa which expires in march
If visa expires then we need to apply new one right ?
Within Canada can we apply for new visa
https://www.canada.ca/en/immigration-refugees-citizenship/services/visit-canada/extend-stay.html
Even if you don't get visa extended by the expiry of the current visa, your parents can stay in Canada under "Implied Status" until the decision is made regarding the visitor visa extension. I will recommend applying a month before the current visa expiry