Hey
You can unlock tasks by double clicking on the task and opening up the task details view. From here you will see an unlock icon on the bottom left hand corner of the task details popup.
Then once you click on that unlock button you will be prompted to log a constraint or an issue. Go ahead and fill in the required fields and hit "submit" then you will notice the task is unlocked when you look in the info column and at the task detail popup. Close the task details and you should be good to go.
Let me know if this was helpful or if you have additional questions.
Thanks and Kind Regards, Sophia