If the tasks are within a single board, you can use the batch actions functionality in monday to update values for multiple items.
If they are either in a single board or across multiple boards, there isn’t a native monday functionality that I know of. Luckily, you can use the Toolkit app Find and Replace functionality to find and replace users or teams.
Thanks @kolaai I have been going board by board and using the filter option to select the owner we want to change and then changing the owner that way but this means going through each board and we currently have 400 boards and will be up to 1500 by the end of our implementation
This Toolkit app looks useful il get a trial and have a place
From your screenshot, I think you are looking at the wrong app. The Toolkit comprises of multiple apps.
For your use case, you need to use the “Find & Replace” app. Select the app in the Apps sidebar as shown in the previously attached image by clicking on the icon at the top left hand corner.