NBA 2K21's demo is here, but it limits you to one created player build per user.
Thankfully, you can get around that with a nifty workaround as spotted by @2KInsiderr.
Here's how to do it.
Reset Demo Progress in NBA 2K21
Essentially, you're going to need to erase your locally stored save data, as well as any cloud versions.
- Head into your console's settings menu
- Go to Application and Saved Data Management
- Then to Saved Data in System Storage
- Delete all files listed under NBA 2K21
- Jump back into your demo.
- Highlight the demo on your Xbox One
- Press start and select "Manage Game and Add-Ons"
- Head to "Saved Data", and delete all the files
- Load back into the game.
Once you've done this, you should be able to create a whole new player build!