To refund your client, you have to log in to your Stripe account. You can then look up the charge in your Stripe dashboard and issue a partial or full refund. Depending on your customer's bank, they will see the refund show up between 5 to 10 business days after you have processed their refund from your Stripe dashboard.
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Go to dashboard.stripe.com/dashboard, then click on the Payments tab in your Stripe Dashboard and click on the payment you would like to refund:

Click on the Refund Payment button at the top right corner:

You can either process a Full refund, or a Partial refund for the client. Refunds usually take 5 to 10 business days to appear on their statement:

You can issue a refund at any time up to 90 days after the charge.  After 90 days, it becomes more likely that certain circumstances may cause a refund to fail - for example if the client's card has expired.

Here's more info on Stripe's refunds.