The HORN valk helm sprite? Well, the horn one is the default sprite, so you just need to delete the files that you installed to see the feathery valk helm, right?
Depends on if you modified your GRF or your data folder.
If you modified the GRF:
Open up the file data.ini, and it should look like
[data]
0=valkhelm.grf
1=abdata.grf
2=sdata.grf
3=data.grfor at least something like this. Then just delete the line "0=valkhelm.grf", and you're done, you don't need to delete the file valkhelm.grf in your RO folder in case you want to have the feathery helm back some time.
If you modified your data folder:
Go to your RO folder and delete the following files:
data\sprite\¾Ç¼¼»ç¸®\¿©\¿©_¹ß۸®Åõ±¸.act
data\sprite\¾Ç¼¼»ç¸®\¿©\¿©_¹ß۸®Åõ±¸.spr
data\sprite\¾Ç¼¼»ç¸®\³²\³²_¹ß۸®Åõ±¸.act
data\sprite\¾Ç¼¼»ç¸®\³²\³²_¹ß۸®Åõ±¸.spr
If the folders are then empty after deleting those files, you can also remove them.