How can I access last year's return, which was filed with TurboTax under someone else's account, and save it in my account?

TurboTax returns can't be moved from one account to another, but if it's your prior-year return, you could ask the owner of the account to provide you with a copy of your return. Here's how they can print it: https://turbotax.community.intuit.ca/replies/2648070