It depends what dampers you have fitted, some can be rebuilt with new inserts, others can't. Check yours to see if there's a large nut on top of the damper - the shiny nut can be seen on top of the black strut on this pic.
Once the nut is removed the outer and insert can be separated like this.
If there is no nut then they cannot be rebuilt and you'll need to replace the complete damper.