Tasks that are created within a trip or contact profile are automatically assigned to that trip or contact, but if you've made a Task outside of those and want to assign them to a trip or contact, here is how!
To assign a trip or contact to a task that has already been created (or when adding a new task from the Tasks page), you can find that task by clicking on “View All Tasks” from the Tasks menu.
This will then open a page of all of your tasks. To edit an existing task that does not have an assigned trip or contact, simply click on the task that you wish to edit and type the Contact/Trip in the “Assign to a Contact or Trip…” field.
Note: tasks that were added to a trip or contact are automatically assigned to that trip or contact and cannot be changed within that trip/contact menu.
In this instance, you would need to delete that task and either re-add it to the appropriate trip or contact (either by adding it directly in the trip or contact, or following the steps above for adding a task via the Task menu or Task page).
If you access that task from within the Task Menu, you can reassign the trip/contact.