There's the potential for doing a Star Alliance Upgrade Award, which is to say using points from a Star Alliance partner airline (e.g. United) to upgrade a cash economy fare to business.
However, for this to work:
- Upgrade space needs to be available; it's not there all the time
- You need to have purchased a specific fare code on your original ticket
You can read more about this
on United's page.
To the second bit, if you're not familiar with fare codes - airlines sell all sorts of different fares at different price points. Cheapest possible, vs. changeable, or fully refundable, or any number of things. So a "cheapest possible" fare may not be eligible for an upgrade.
It's also worth noting that Air Canada stopped doing Star Alliance Upgrade Awards sometime during the pandemic and apparently has not reinstated them. There's discussion about this on the Air Canada forum here on FT.
Hope that helps.