Under the cog ⚙️ icon at the top right, select "Profile Info" (4th option down) or just link directly to https://www.furaffinity.net/controls/profile/

The "Display Name" option can be updated with a custom name (every 7 days); the sort that y'can see on people's profiles where both names aren't the same.

It's still not possible to change your underlying "User Name", unfortunately; presumably due to the same data indexing issues inherited from two decades ago.

Good enough for me, thank you for the information!