From my warranty card:
Repairs can only be made by our authorized Service Station.
This lifetime warranty becomes invalid for any gun which is handled
improperly, modified, or disassembled. Should any defect appear,
package the gun carefully in a solid shipping box (the original
Diana/RWS box is not a shipping box) and return prepaid to:
RUAG Ammotec USA, Inc
DBA: Dynamit Nobel RWS Inc
81 Ruckman Road
Closter, NJ 07624
Phone: (201) 767-1995
They also say to include a check or money order for $10 to cover handling.
Include the name, address, and phone number of the shipper in the package along with a note explaining the defect found.