Gifting and deducting credits

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

If you go to the 'Menu' icon in the top right...

then select 'Contacts' from the options

Select the client's profile

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

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