<script>

let list = [1, 2, 3];

const changeList = (index) => list[index] *=2

$: console.log('list', list)

</script>

changing item from inside the each block: <br />

{#each list as item}

<button on:click={() => item *= 2 }>{ item }</button>

{/each}

<br /><br />

calling a function from inside the each block that changes the item: <br />

{#each list as item, index}

<button on:click={() => changeList(index) }>{ item }</button>

{/each}