If you are an assignee of the Task, click on
Unclaim to unclaim it from the current claimant user. Once you have unclaimed the Task, you will be able to claim it.
Note: in order to be able to unclaim a Task you need to be an assignee of the Task. If you are an assignee but you still cannot unclaim the Task, your User Type probably does not include permission to unlock files. Please refer to this article for more details:
Which permissions need to be set in the User Type to be able to unclaim a Task from another user and unlock files in Explorer?Adding the missing permission to your User Type will allow you to unclaim a Task. If changing the User Type is not possible, ask the claimant of the Task to unclaim it or ask a user who has the needed permissions to unclaim it for you.