Either plane down the high side with a sharp scraper, feathering it back about a foot.
OR, use floor leveler over the subfloor on the low side to feather it out.
The floor leveler comes in a powder, you mix it with water and it sets very quickly. Just trowel it on and scrape the ridges, left from the edge of the trowel, before it sets completely.
I'd go for the floor leveler. The scraper is hard to be precise.
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