You can unblock a user account from your profile

• Go to your profile and click on profile menu ( 3 dots) in the righthand corner and click blocked users.
• Find the user to unblock and click the X to unblock.

Or you can unblock users from the account settings

• From the main timeline screen, click on the person icon in the top righthand corner.
• Select Settings, then Account and choose Blocked Users
• Find the user to unblock and click the X to unblock.