These instructions assume that the copper pour area is still present.  If it has been deleted, you will need to create a new pour area around the old one, then re-pour this which will link the old fill to the new area.

If you find you need to remove or alter the copper that has been poured, you should remove the copper by selecting it or the associated copper pour area, right-clicking and choosing 'Clear Copper' from the menu. This will 'unpour' the copper pour area, allowing you to make the necessary modifications to the design (perhaps some extra tracking has to be added), before re-pouring the copper.

If you simply want to change where the copper is poured, you can edit the copper pour area without clearing the copper first. The area 'knows' which copper shapes belong to it, so when you re-pour the adjusted area it will clear away all the previous copper shapes first.