You have purchased a domain from Google domains, and now you want to cancel it and want your money refund. But make sure that you take this action within 5 days of the purchase of the domain name. Below is the screenshot of the Google Domain Refund Policy page:

Cancel Google Domain Process
To cancel recently purchased Google domain click on the following link Google Support and choose chat, email or request a call option. By talking with the support executive, you would be able to cancel your domain and can get your money back within 5 days of purchase.
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Hope that helps.
sir,
your answer to the question "how to cancel domain & get refund" I tried to follow that but iIfailed to do so. I am not a computer savvy & as a lay man I found it complicated.
by mistake I paid twice in last three days. can you please help me on this to cancel domain & get refund. I am prepared to pay your professional charges.