# cartLinesUpdate

Updates merchandise lines in an existing cart.

## Authentication

> **Public Access**: Accessible with the public API key

## Mutation Structure

```graphql
mutation {
  cartLinesUpdate(id: String!, lines: [CartLinesUpdateInput]!) {
    cart {
      # Cart fields
    }
    userErrors {
      # UserError fields
    }
  }
}
```

## Input Arguments

| Argument | Type | Required | Description |
|----------|------|----------|-------------|
| id | String | Yes | The ID of the cart to update lines in |
| lines | [CartLinesUpdateInput] | Yes | A list of merchandise lines to update in the cart |

## Return Type

[CartLinesUpdatePayload](/reference/storefront/1.3.0/objects/cart-lines-update-payload) - Payload returned by the cartLinesUpdate mutation. Contains the updated cart and any errors that occurred during the update operation.

## Example

```graphql
mutation {
  cartLinesUpdate(
    id: "eyJhbGciOiJIUzI1NiJ9..."
    lines: [
      {
        id: "line-123"
        quantity: 2
      }
    ]
  ) {
    cart {
      id
      lines {
        id
        quantity
      }
    }
    userErrors {
      field
      message
    }
  }
}
```

## Related Types

- [CartLinesUpdatePayload](/reference/storefront/1.3.0/objects/cart-lines-update-payload)
- [CartLinesUpdateInput](/reference/storefront/1.3.0/inputs/cart-lines-update-input)