You first need to delegate the task to you (if you've enabled the delegation when configuring the flexi task) and then you will be able to do that.
Hope this helps
Flexi task has an option to Allow delegation. However this works only if the approvers are delegating the task to someone else. Hope this helps!
In addition to Caroline's comment, another strategy is to assign the task to a SharePoint group created for this specific task so that there is back up for the primary approver.