The first step to make is to ensure that your storefront is set up to accept selling in multiple currencies, under Shopify Payments.
Please navigate to:
Under the 'Shopify Payments' section at the top, click Manage:
In the Multiple currencies section, enable the currencies that you want to support from the list of accepted currencies - remember to hit Save, when you're done
Once this step - selecting and saving your preferred currencies - is done, your Capital theme will be immediately able to display these options for you in the Currency Selector, allowing your customer to freely select their currency to shop in:
This Selector option can be added to both your Header and Footer, allowing the customer to change their currency immediately and accessibly.