Gifting and deducting credits

From your 'My Hub' page (please see below)

On the left hand side of your account, you will see the side menu where you can select 'Contacts'.

Select the client's profile

Then Click 'Financials' from the list below the client's profile picture followed by 'Credits'

Here you will be able to both 'Gift' and 'Deduct' credits to that client's account.