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You cannot extend a US visitor's visa while you are in Canada, but you can apply for a new one at a US diplomatic mission in Canada.
You can also re-enter the US with an expired visa if you've been in Canada for no more than 30 days, thanks to automatic revalidation. This seems unlikely to help you since you have indicated that you are now in Canada and wish to enter the US later this summer, which suggests that you will have been outside the US for more than 30 days.
Be careful not to confuse visa validity with duration of stay. The former is indicated by the "expires on" date on the visa sticker. The latter is usually written into your passport on the admission stamp. For more information, see What the Visa Expiration Date Means.
To get a new visitor visa, and to be admitted to the US as a visitor, you will have to convince the visa officer and immigration officer, respectively, that you do not intend to immigrate to the US and do intend to meet the requirements placed on visitors. If you have recently spent a significant amount of time in the US, this will be more difficult.