Navigate to your User Settings and select Edit to update your Discord tag.
You'll see that your Discord Tag is customizable after that.
Make a new, four-digit Discord tag for yourself between 0001 and 9999. (You may have 0001 as well, yeah)
Click Save after entering your current password.
A confirmation popup will appear informing you that canceling your Nitro subscription will result in the randomization of your personalized Tag once more.
To complete modifying your Discord tag, click Change Discord Tag.