ummm...... how do i ugrade that horn ?

Been posted a bijjiollion times but
Get a diamond and the Beljuril Gem. Go to the storekeeper thats by that wooden platform with the Paintseller and the guard. Ask him about what else he does, he'll tell what you need to upgrade it and upgrade the gem.

<font color="gold">SPOILERS
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A Diamond can be picked up from several locations, while there only excists two Beljuril Gems in the whole game. I only know for sure where one is located, which is in a hen in the Umar Hills. Talk to the guy just north of the town, and he'll tell you about the farmer who owns the hens, and explain everything to you. ;)

The other one I know is somewhere inside Firkragg's Dungeon, but not exactly where.</font>

Spoiler
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In the secret room with all the golems. It is in the same cache as the Heartseeker.