Firstly, let's start by updating the value of the 'NHS Test'

• Goto 'Stock Menu -> Option 2'
• Open the stock record for 'NHS Test' (this is usually code 'N')
• Update the amount to the new value
• Press 'Ctrl+Enter' to save

Secondly, we need to update the invoice payment values for vouchers

• Goto 'Till / Invoice' menu
• Goto 'Maintenance -> Payment Method / Voucher Values'

• Click on the button shown below to alter the values shown

• Update the value in 'Inv Pay Val1'

• Press 'Ctrl+Enter' to save
• Repeat this process for each voucher value that needs updating