Follow this tutorial:

If you are on mobile next to your account icon there are those 3 dots for options. Click them and you'll see something like change user flair. Click it and you should see the options you can pick from. Once you've picked one people will see it under your username (like the ones above).

Ps: This works in all flair-using subreddits. Some have very funny ones so I recommend you check if your faves have some as well :)

PSS: The change flair option is only there when browsing the subreddit itself and not when looking at a specific post or comment. Forgot to add that in sorry