You can transfer your attendee's funds from one signup or registration to another. Transferring funds allows you to combine registrations for people who accidentally registered twice, avoid extra payments, and save on Stripe fees.
From the registration where the payment was made, click Issue refund.
Refund to Other, not the original payment method. Include a note about where you're applying the funds along with the new registration number, so it's easy to track down.
Open or create the registration receiving the funds, then click Apply payment.
Again, choose the Other payment method, not the original method of payment. In the note, enter the signup, person, and registration number the money was originally paid.
Don't forget to send the receipt to the Registration Contact, so they know the funds transfer has been made.
You can see the transactions on the Payments tab of each signup or the Payments page.