how to search for the value with a certain index in an array and modify its value?

I have two arrays, the first contains two numbers and the second 12 values.

firstArray = [0, 6];

secondArray = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"];

I want to take the numbers of the first array and and check that index of the second array matches and then change the value of the matching ones.

In this case the output would be

secondArray = ["blank", "February", "March", "April", "May", "June", "blank", "August", "September", "October", "November", "December"];

I was trying to for-loop for getting the values in firstArray but I'm gettin the indexes and not the value

    for (let i = 0; i < this.state.lastIndex.length; i++) {

    }

hay How can I do it? Is there any way to make it more direct than with a for-loop?

thanks for your help