You can follow the instructions here:
Infinity MileageLands- Mileage Redemption - EVA Air | North America (English)
https://www.evaair.com/en-us/infinity-mileagelands/mileage-award-program/mileage-redemption/

You can techincally check the availability of free tickets without having the miles but the interface is pretty bad as you have to enter the specific dates.
For upgrades you need to have already bought a ticket before you can check. When checking for upgrades, it will either say available or waitlist, if there is a waitlist then the upgrade may or may not go through.
Alternatively, sites like Expert Flyer (a paid third-party service) may be able to check for availability for you. (with a subscription fee.)

I just bought for an elderly family member a Premimum Economy ticket and successfully upgraded her to Business Class round trip.
The PE ticket is not cheap but at least if the upgrade doesn't go through she would have at least PE. That was my plan. It worked better than I expected as the upgrade was instantly cleared both ways.

If you have Citi Thank You Points the transfer time from Citi To Eva is just about 1-2 days from my experience.