When a ticket is locked by a user, it is possible to unlock it using a specific action.
Preview of a locked ticket
To unlock the ticket, you must select it by checking the box on the right at the end of the line and choose the action "Unlock the files" in the "Action" list.
When the action is selected, click "Apply".
Confirm unlocking by clicking "Yes, unlock".
The right "Unlock a ticket" is not assigned by default to agents, this right must be added by modifying the rights of the user or one of his teams.