There is nothing to rebuilding a jack. The hardest thing is making a big spanner to undo the large nut on the top or finding a shifting spanner big enough to do the job. There is nothing inside other than a cup, shafts and O rings. Pull it apart and lay it out and clean and re assemble in reverse order replacing old O rings with new and replace the cup. Fill with hydraulic jack oil and bleed air out by lifting to the full extent and pushing back in. You might have to do it a couple of times you may have to top up the oil while doing this and have the screw on the top done up as well and loosen it while pushing the extensions in. Someone else might add more to this.
Cheers
Rod