You have blocked someone on Twitter and are now wondering how you can unblock that Twitter account.
In this quick step-by-step tutorial you will learn how to do that.
Let’s start:
How to Unblock a Twitter Account
Step 1: Navigate to the Twitter profile of the blocked user.
Step 2: Next, select “Blocked” from the drop-down menu. Finally, choose Yes or Unblock on Twitter to indicate if you want to unblock the account.

So this is how you can easily unblock any Twitter account.
Reference: https://help.twitter.com/en/using-twitter/blocking-and-unblocking-accounts
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