nvm i just found an answer...
i meant with reset completely new nether seed but its not possible without changing the overworld 
Posts by eZkYroX
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i just tried to delete both to be sure and its the same result
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still nothing
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yes -1 not 1
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nothing changed
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I can't reset the nether, i tried multiple times to delete DIM-1 and its always the same.
Vanilla 1.18.1 custom world (played in singleplayer before but never entered nether)