To change your Minecraft name in the Java Edition, head to Minecraft.net and log in with your account. Once done, head to the profile page, look for the Minecraft: Java Edition box, and click ...

Select “Profile Name” Change to the desired name and save This name change doesn’t effect the Gamertag found on the Xbox account, and will only be shown when playing Java edition.

How do I find my Minecraft username? On the change name screen, you can see your previous name located right after the field “Current Java/Bedrock Profile Name”.