If a Task should be linked to a Project or a Booking linked to a different Project, go to the Toolbox > Projects tab > select the Project and drop it on top of the Task/ Booking and Confirm Changing Project / Task:
When you move a Booking between different Projects, there is a dialog asking "Do You Want To Reset Rates?".
It enables you to either keep the rates for that Booking or to apply the new Project's rates, e.g. in case there is a Rate Card in it.
That dialog will not show in the case of adding a Project to a Task, but still shows when changing the Projects of a Booking.