Yes! Once you have the gift card, click here and log in to your existing Calm account. Enter the redemption code on your card, and the new gift subscription will be active on your account.
If you currently have an active subscription on your account, you will need to ensure the auto-renewal has been canceled and then wait until the subscription expires before redeeming the gift card. Please see this support article for more details about how to cancel the auto-renewal of a subscription.