If my unit is already mighty, can I still use Fiora legend ability?

No, you cannot trigger the Fiora Legend ability if a unit is already at 5 or more Might.

According to Rule 709: "A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."

As clarified in RiftJudge FAQ #9792, if a unit is already at 5 or greater Might, it is already considered "Mighty" (Rule 708). Increasing its Might further does not change its status from "not Mighty" to "Mighty," so it does not trigger the Fiora Legend ability.