If you have administrator permissions, you can assign a task to yourself or another user it cannot be completed by the original assignee.
From the left-hand sidebar menu, go to Tasks
Turn off My requests only to show all requests
Open the task you'd like to reassign
Click the pencil button next to the assigned user in the Assignee field
Choose the user(s) you'd like to assign the task to from the drop-down
Click the green checkmark to confirm your choice; click the red cross to cancel
Note
Click ✕ on the user name chip to remove a user from the task completely. If you do this, make sure you have assigned a user to replace them to avoid the task becoming unassigned.