Given an array, if you know the index of an item you can replace its content using a simple assignment:

const items = ['a', 'b', 'c', 'd', 'e', 'f']
const i = 2

items[i] = '--NEW-ITEM--'

console.log(items)
//[ 'a', 'b', '--NEW-ITEM--', 'd', 'e', 'f' ]

If you don’t know the index of the item, you might need to first find the index of the item in the array.