Once a task is created, it has owners, reviewers, and authors assigned to it. You can change any one of these from one user to another by using the Transfer Task tab. It can be found by clicking the Builder Settings Menu (top right cog icon) > Search & Replace > Transfer Task. Once in the tab, you can select which of Owner, Reviewer, or Author you would like to replace by clicking the drop down menu under replace. Then, select the 'from' user from the drop down menu under Select Source User. From here, select the 'to' user from the drop down menu under Select Target User(s), there can only be one target user. Click the search button, and all of the tasks that have the selected source user (and not the target user) as the owner/reviewer/author will display in a list at the bottom of the page. Select the check box next to each of the desired tasks to transfer and click the 'replace X items' button.
A simple popup will be displayed to confirm the selections made. Once 'yes' is clicked, the Owner, Reviewer, or Author will be transferred from source user to target user.