I've looked into this already, as mine is getting done very soon. You can either take it to an auto upholsterer who will remove and replace panels in the seat cover, or buy new whole seat covers yourself from VW. All the parts that make up a seat are available individually.
The back support cover which includes the bolster fabric that is always rubbed upon entry/exit is 90 pounds from VW. The piece of foam required if your foam has sagged, lost shape or broken is around 60 pounds from VW.
I have spoken to Bells in Guildford who do my bodywork , and they said it is quite a job getting a new cover on, as it fits so tight to the seat, so you may want to get it fitted. They quoted me an hour and a halfs labour to do foam bolster and put cover on.
Ring your local VW dealer's parts department and they'll quote exact prices and part numbers etc.
