Those are very hard to replace as they are fragiel and awkard to deal with. You first have to put them in an oven at a low temperature untill the sealant allows you to remove the cover. You can probably find this information on an enthusiasts forum as msost custom projects have been done by enthusiasts and that is your best chane for finding the temperature and time to put it in the oven to break the seal. It will damage the light if it is too high a temp and in there too long.
Then you have to find out how the rings are mounted and replace and remove them.
A good source on how to do all of this is the tech department for the manufacturer of the headlight.
Althoug, stay anonymous as you can risk your warranty may be voided.