Before the flight, you can purchase a voucher in the Lufthansa/Telekom Ground Shop, which you can then redeem on board via the FlyNet® portal. Click here to go to the ground shop:
Lufthansa Ground Shop (telekom.de).
If internet is not available on your flight, you can also redeem the voucher on a future long-haul flight with the Discover Airlines or Lufthansa FlyNet® service on board. The voucher is valid for the calendar year of purchase and the three following calendar years and can only be redeemed once on a flight segment (e.g. Frankfurt-Windhoek) with the Discover Airlines or Lufthansa long-haul fleet equipped with the FlyNet® system.
Vouchers that you have received elsewhere (e.g. as part of promotions) may have different terms of use or validity than the tariffs on board or in the Ground Shop.