1) customer can select another variant of the same product. You can see a demo of this here: https://docs.richcommerce.co/getting-started/how-to-activate-exchanges-on-your-account
2) if you are on our Premium Plan or higher you can then simply click "Exchange Order" inside our dashboard and the exchange order with all details is created automatically. See a demo here: https://docs.richcommerce.co/getting-started/exchange-orders-one-click
At the moment we create "Free" Exchange Orders only so we don't automatically charge the customer for any price difference. Since variants often have the same price this is usually not an issue at all.