If you add a new resource assignment to an activity that is in progress, you should use an offset to indicate that the additional resource will be in the future. If you don't use an offset, the resource will be prorated and only the calculated remaining amount is added to the forecast amount.
You must not edit existing resource assignments. In order to delete a resource assignment or reduce hours, you must insert a new line and enter a negative value to indicate a change. In order to increase hours, you must insert a new line and enter a positive value to indicate a change. You also have to put the change number in the cost class field on the resource assignment.