On the final level you will find a room that has to be opened by a power node.

You open that room and when you go further there will be a room with a store to your left and a bench to you right.

Go up the stairs and save then restart from the save point.

Then go to that room and you will find a power node in it.

Then upgrade then save then do it again!

It will keep on repeating to come. Just keep doing that until everything is upgraded!