Thai Airways also allows the passenger to upgrade the flight through their official website. So let's move forward to know more about How do I qualify for an upgrade on my flight? Yes, go through the below steps:

Steps to upgrade the Thai Airways Flights
Visit the Thai Airways website, click on the manage my booking option.
Then enter your reservation number and the passengers last name.
Click on the search booking option and then select your reservation from the other page.
Now if you are upgrading your flight ticket using miles then click on the upgrade option.
Choose the business class from the select class option.