When a user account is locked, they will get the following message when trying to log in.
To unlock an account that has become locked, go to Admin > User Management and then locate the user whose account is locked.
Click on the burger menu (highlighted yellow in screenshot below) next to their name and you will see an option "Unlock Account".
Once you click, "Unlock Account" you will get a final prompt for you to confirm the unlock.
If you're sure you want to unlock the account, press the "Unlock" button and you'll then see a confirmation message.
Unlocking the account will also reset the users number of incorrect login attempts to zero.