if i debuff a 5 might unit to 2might. and then end my turn. it goes back to 5 might. can fiora tap her legend because it went back to 5?

Yes. When the debuff expires at the end of your turn, the unit's Might goes from 2 back to 5 — that's the exact transition where a unit "becomes Mighty" (Rule 709), so Fiora, Grand Duelist's ability triggers during the Expiration Step. As long as Fiora is readied, you may exhaust her to channel 1 rune exhausted (FAQ #742). If she's already exhausted, the ability still triggers but you can't pay the cost.