Welcome to FT.

Y, B or M is what you're looking for in terms of upgradeable fares. There will need to be upgrade inventory, seats that are desigated upgradeable, for you to use miles. Usually, it is easier to call into reservations and have them help you find a flight and fare that will get you what you need.

One suggestion, NEVER book a Y, B or M fare before checking to see if upgrade seats are available.