Can I cancel a gift card that was issued by mistake?

Yes, as long as it hasn't been redeemed yet. In Plugins → Gift Cards, find the card under Issued cards and click Disable. Only Active cards show this button. Then issue a new card with the right amount using Issue a gift card.

Use the status filter to see Active, Redeemed and Disabled cards. A card that was already redeemed has become store credit in the customer's wallet, so it has to be handled from the wallet instead.